Anti-Microbial Agents (Antibiotics & Chemotherapeutic Agents) Antibiotics: An antibiotic is a chemical substance derived from or produced by various species of microorganisms which is capable in small concentrations of inhibiting the growth of other m.o (bacteriostatic) or killing them (bactericidal). Those of the greatest use are derived from Bacillus, Penicillium, Streptomyces, or synthesized (the synthesized are called chemotherapeutic agents, while the naturally produced ones from m.o. are called antibiotics). The anti-microbial agents counter act against bacteria, viruses, or fungi. The main concerns of these lectures will be the antibiotics and chemotherapeutic agents against bacteria. The disinfectants are anti-microbial agents but lack the selective toxicity (i.e. being harmful to both body tissues and m.o.).
Mechanismsof Action Ideally the drug should have “ Selective Toxicxity” which means harmful to a parasite without being harmful to the host. This is relative rather than absolute. The action may be conducted through receptors for drug attachment or inhibition of biochemical events essential to the parasite. Generally, the mechanisms of anti-microbial drugs against bacteria are mediated through:1. Inhibition of cell wall synthesis.2. Alteration of cell membrane permeability or inhibition of active transport across cell membrane.3. Inhibition of protein synthesis.4. Inhibition of nucleic acid synthesis. .
First: Antimicrobial Action Through Inhibition of Cell Wall Synthesis Examples: 1. Penicillins *** 2. Cephalosporins*** 3. Vancomycin*** 4. Bacitracin 5. Ristocetin 6. Cycloserine *** = Important
Penicillins: These drugs are derived from molds of genus Penicillium (P. Notatum). The penicillins have a nucleus which is called 6-Aminopenicillanic Acid (6-APA). By coupling of the free amino group of this nucleus to the free carboxyl group of different radicals, unlimited penicillin compounds are produced or synthesized. The penicillins (6-APA) have two rings: a. Thiazolidine Ring. b. Beta-lactam Ring ( the site of action of certain enzymes called penicillinase or Beta-lactamases that can break down this ring and consequently render the penicillin inactive,i.e., without antibacterial activity). If the Beta-lactam ring is cleaved by penicillinases (Beta-lactamases) the penicillin structure is converted to Penicilloic Acid (See the Fig.).
Mechanisms of bacterial resistance to penicillins: 1. Production of penicillin destroying enzymes (Beta-Lactamases) which are of more than 100 plasmid-controlled types. These enzymes break down (open) the beta-lactam ring of the penicillin (See Fig.). Such enzymes are produced by Staphylococcus aureus, Haemophilus influenzae & Neisseria gonorrhoeae. 2. Lack (absence) of PBPs receptors or inaccessibility to them. This is under chromosomal control (chromosomal mutation). 3. Failure to activate autolytic enzymes in the cell wall which could lead to inhibition rather than killing of the m.o. 4. Failure to synthesize peptidoglycan as L-form bacteria or Mycoplasma (naturally have no peptidoglycan).
Mechanism of Action of Penicillins : 1. Binding of the drug to cell receptors called penicillin-binding proteins (PBPs). Each bacteria have about 3-6 PBPs / cell. These receptors have different affinities for the drugs and some of them are enzymes of transpeptidation. 2. Then transpeptidation is inhibited and peptidoglycan sythesis is blocked. 3. Removal or inactivation of an inhibitor of autolytic enzymes in the cell wall which activates the lytic (lysis) effect of them. N.B.: The penicillins lack toxicity to animal cells since they have no cell wall with peptidoglycan. The susceptibility of bacteria depends on amount of peptidoglycan, numbers of PBPs, and activity of autolytic enzymes.
Types of penicillins: 1. Penicillins active against gram- positive bacteria (not or limited against gram negative), sensitive to penicillinases, and are acid labile (e.g., destroyed by gastric juice). Examples: penicillin-G as sodium or potassium salts (other names: natural penicillin , crystalline penicillin or Benzyl penicillin ), procaine penicillin, & benzathine penicillin ( long acting penicillin given I.M. every 3-4 weeks for prophylaxis against certain infections as Streptococcal ones). 2. Penicillins active against gram positive bacteria, resistant to penicillinases and acid stable ( can be taken orally). Examples: Oxacillin, Cloxacillin, dicloxacillin, Flucloxacillin & Nafcillin . 3. Penicillins active against gram positive bacteria, and acid stable, but sensitive to penicillinases. Example: Penicillin-V (have weak effect) and Azidocillin.
Cont./……Types of penicillins:4. Broad (wide)-spectrum penicillins which are active against both gram-positive & negative bacteria, and are penicillinases sensitive. Some of these penicillins are acid stable as ampicillin and amoxycillin (similar to ampicillin but more rapidly absorbed from the intestine), and some others are acid sensitive as Carbenicillin & ticarcillin.5. Penicillins protected from destruction by penicillinases. Examples: Amoxycillin + Clavulanic acid (i.e. , the latter protects the former from destruction by penicillinases).6. New penicillins with more potent effect on gram negative than gram positive bacteria specially those which are resistant to other drugs (e.g. , Pseudomonas aeruginosa). Examples: piperacillin, azlocillin, & mezlocillin (which are more effective than carbenicillin & ticarcillin on these m.o.).
Beta-Lactamases: There are more than 100 types which have 3 categories of effects against different anti-bacterial drugs: 1. Those produced by Staph. aureus, H. Influenzae, & E. Coli that destroy sensitive penicillins, but not cephalosporins. 2. Those produced by Pseudomonas, and Enterobacter that hydrolyse both sensitive penicillins & cephalosporins. 3. Metallo- beta lactamases destroy certain drugs which are resistant in the 1 & 2 categories. Example: Carbapenems (beta-lactam drug) resistant to penicillinases and cephalosporinases, but sensitive to metallo- beta lactamases. Beta-Lactam Drugs: 1. Penicillins. 2. Cephalosporins. 3. Monobactam. 4. Carbapenems. N.B. : Methicillin is a penicillin drug produced in 1960. It is resistant to beta-lactamases specially those which are produced by Staph. aureus. This drug is not any more in use because of its nephrotoxicity. However, it is useful to differentiate Staphylococci into methicillin-resistant & methicillin-sensitive.
Beta-Lactamases: There are more than 100 types which have 3 categories of effects against different anti-bacterial drugs: 1. Those produced by Staph. aureus, H. Influenzae, & E. Coli that destroy sensitive penicillins, but not cephalosporins. 2. Those produced by Pseudomonas, and Enterobacter that hydrolyse both sensitive penicillins & cephalosporins. 3. Metallo- beta lactamases destroy certain drugs which are resistant in the 1 & 2 categories. Example: Carbapenems (beta-lactam drug) resistant to penicillinases and cephalosporinases, but sensitive to metallo- beta lactamases.
Probenicid Penicillins are excreted by glomeruli (10%), and renal tubules (90%). This can be blocked by probenecid to achieve higher systemic and CSF levels (e.g., in the treatment of meningitis).
Cephalosporins: These drugs are beta-lactam drugs. Mechanism of action: They act on the cell wall synthesis of bacteria in a similar manner to penicillins: 1. Bind to PBPs (drug receptors similar to those of penicillins). 2. Block transpeptidation process of peptidoglycan. 3. Activate autolytic enzymes that can produce lesions in the cell wall and resulting in bacterial death. Mechanisms of drug resistance: The resistance to cephalosporins can be attributed to: 1. Lack of PBPs. 2. Poor permeation of bacteria by the drug. 3. Production of beta-lactamases that can destroy some cephalosporins.
Basic structure of cephalosporins The basic structure of cephalosporins is comparable to that of penicillns. The nucleus of cephalosporins is called 7-amino- cephalosporanic acid and by attachment of various R side-groups enormous array of drugs are generated which are grouped into FIVE groups or generations.
First- Generation Cephalosporins: Examples: Cephalexin*, Cephradine*, Cephalothin, Cephapirin, Cefazolin & Cefadroxil*. These drugs have the following antibacterial activity: 1. They are very active against gram positive cocci, except enterococci and nafcillin-resistant staph. 2. They are moderately active against some gram negative rods as E. Coli, Proteus & Klebsiella. 3. Anaerobic cocci are often sensitive to these drugs, but Bacteroides fragilis is not. * Some of these drugs are oral agents and can be absorbed from the gut. They are used to treat urinary and respiratory tract infections. Cefazoline is a choice for sugical prophylaxis. These drugs can not penerate the CNS, therefore, they are not used in the treatment of menigitis.
Second-Generation Cephalosporins: Examples: Cefamandole, Cefuroxime, Cefaclor*, Cefoxitin, Cefprozil* (promoted in 1986). These drugs have the following anti-bacterial activity: 1. All are active against m.o. covered by the first generation. 2. They have extended coverage against gram-negative rods including Klebsiella, & Proteus, but not P. aeruginosa. 3. Some oral drugs can be used to treat sinusitis and otitis caused by Haemophilus influenzae, including beta-lactamase producing strains. 4. Some are active against B. fragilis and used in mixed anaerobic infections. 5. Can be used in combination with aminoglycosides for sepsis in immuno-deficient/-suppressed patients.
Third – Generation Cephalosporins:Examples: Cefotaxime, Ceftriaxone, Ceftazidime, Cefoperazone, Cefixime*, Ceftibuten*, Cefdinir*, Ceftizoxime.These druges have the following anti-bacterial activity:1. Decreased activity against gram-positive bacteria; enterococci often produce superinfection during their use.2. Have enhanced activity against gram-negative rods; including the succeful effect of ceftazidime and cefoperazone against P. aeruginosa. Therefore, are very useful in the treatment of hospital acquired (nosocomial) gram negative bacteraemia.3. Used in combination with aminoglycosides in the treatment of infections in immunocompromised patients.4. Ceftazidime is useful in the treatment of melioidosis ( P.pseudomallei infection).5. Have the ability to reach the CNS and appear in the CSF (except cefoperazone). Therefore, are used in the treatment of gram -negative rods meningitis.6. Some are the drugs of choice in the treatment of gram negative rods sepsis (e.g. I.V. Administration of cefotaxime, ceftriazone, or ceftizoxime.* = Can be used orally
Fourth – Generation Cephalosporins:Examples: Cefepime (maxipime), cefclidine, celuprenam, cefpirone, cefquinome. These drugs have the following anti-bacterial activity:1. Have an enhanced activity against Enterobacter and Citrobacter species that are resistant to the third generation cephalosporins.2. Cefepime has a comparable activity to that of ceftazidime against P. aeruginosa.3. It has also comparable effect against streptococci and nafcillin-susceptible staphylococci to the third generation cephalosporins (except ceftazidime which has a weaker effect).
Fifth- Generation Cephalosporin Examples: Ceftobiprole, and Ceftaroline; the former has a better anti-Pseudomonas effect and less resistance development than the latter.
Monobactams: They have beta-lactam ring and are resistant to beta-lactamases. They are active against gram negative rods, but not against gram positive bacteria or anaerobes. Example: Aztreonam which resembles aminoglycosides & is given I.V. or I.M. Patients allergic to penicillins can tolerate aztreonam. However, superinfection with staphylococci and enterococci can occur during the use of aztreonam.
Carbapenems: These drugs are related to beta-lactam drugs. There are different types: 1. Imipenem: This is the first drug of carbapenems. A. Has good activity against many gram negative rods, gram positive organisms, and anaerobes. B. It is resistant to beta-lactamases, but is inactivated by dihydropeptidases in renal tubules. So, it is administered together with a peptidase inhibitor (Cilastatin). C. Can penetrate the body tissues and fluids well including the CSF. It is given I.V. every 6-8 hours. D. May be indicated in the treatment of infections resistant to other drugs as P. aeruginosa. This m.o. develops resistance to imipenem rapidly, so this drug should be used in combination with aminoglycosides. E. It has adverse effects including vomiting, diarrhea, skin rashes and reactions at infusion sites. In renal failure, the use of this drug may lead to seizures. Patients allergic to penicillins may be allergic to imipenem as well.
2. Meropenem: It is similar to imipenem in pharmacology and antimicrobial spectrum of activity. However, it is not inactivated by dipeptidases and is less likely to cause seizures than imipenem. Examples of commercial names: Cefotaxime : Claforan Cefuroxime : Zinacef Cephalexin : Keflex, Cephalexin (S.D.I) Cefamandole : Mandol Amoxicillin + Cloxacillin : Ampiclox Amoxicillin + Calvulanic Acid : Amoxaclave
Glycopeptides 1. Vancomycin: Vancomycin is produced by Streptomyces orientalis. It is poorly absorbed from the intestine and is given I.V. in serious infections. The drug inhibits early stages in cell wall peptidoglycan synthesis and is bactericidal. Antibacterial activity: 1. It can markedly kill staphylococci (including those causing endocarditis or nafcillin-resistant) , some clostridia and some bacilli. 2. Enterococcal sepsis or endocarditis specialy when combined with a penicillin. Developm- ent of entrococcal resistance to vancomycin is problematic. 3. Used orally in the treatment of pseudomembranous colitis (see Clindamycin).
2. Teicoplanin: Teicoplanin has a structure similar to that of vancomycin. It is active against staphylococci (including nafcillin-resistant strains), streptococci, enterococci, and many other gram-positive bacteria. The drug is taken once daily (has a long half-life).
II. Anti - Microbial Action Through Inhibition oCell Membrane Function: Examples : Amphotericine B, Imidazoles, Nystatin, Polymyxins, Triazoles. Polyenes and Azoles: Polynes include amphotericin B and Nystatin), imidazoles and Triazoles(Azoles). These drugs are active against eukaryotic cells (fungi) and not against prokaryotic cells (bacteria). This is because the cell membrane of eukaryotic cells contains a substance which is called sterol which is not present in prokaryotic cells. The polyenes/Azoles combine with the sterol making a complex called sterol-polyene/azole complex which damages the cell membrane by forming craters or vesiculation leading to interference with cell membrane functions.
Amphotericin- B :It is a complex antibiotic polyene produced by Streptomyces. It inhibits several pathogenic fungi by binding with their sterol. It can be used for treatment of disseminated coccidioidomycosis, blastomycosis, histoplasmosis, cryptococcosis & candidiasis. It can be given I.V. & intrathecally.Toxicity of Amphotericin – B : Fever, chills, nausea, vomiting, nephrotoxic (renal failure), hypokalaemia, hypocalcaemia & anaemia. Therefore its use is quite restricted. Liposomal amphotericin is less toxic but expensive.Synergism: with Flucytosine
Nystatine : This drug is quite toxic to be used prentrally. So, it is used topically on the skin for superficial infections as candidiasis. Imidazoles: This drug is also anti-fungal. It inhibits the biosynthesis of cell membrane sterol (ergosterol), impairs its integrity and increases its permeability. There are 3 types of imidazoles: 1. Clotrimazole (oral, cream & vaginal sup.); for candidiasis. Commercial name: Canesten. 2. Miconazole (2% cream, oral, I.V.) for dermatophytosis & vaginal candidiasis. 3. Ketoconazole (oral): for mucocutaneous & vaginal candidiasis, and paracoccidiomycosis. Adverse Effects : nausea, vomiting, headache, skin rashes, increased transaminase, inhibition of adrenal steroid synthesis and gynaecomastia.
Triazoles: 1. Fluconazole: Enters CSF; used for candidiasis, and CNS Cryptococcosis. 2. Itraconazole: Used in liquid form for blastomycosis, histoplasmosis, and aspergillosis. 3. Voriconazole: Used orally and I.V. for candidiasis, cryptococcosis, and aspergillosis and other filamentous fungi.
Polymyxins: There are A, B, C, D, & E types, however, only B and E (colistin) can be used clinically. They act on the phospholipids & lipopolysacchari-des of the cell membrane. They coat the cell membrane and destroy its active transport functions (act as cationic detergents). They are bactericidal drugs against gram negative bacteria including Pseudomonas aeruginosa which is resistant to other drugs.
III. Anti-Microbial Action Through Inhibition of Protein Synthesis: Examples: Aminoglycosides (streptomycin, neomycin, kanamycin, tobramycin, gentamicin , amikacin & netilmicin), Chloramphenicol, Macrolides (Erythromycins & Lincomycins), Tetracyclines , Fusidic acid and Linzolide.
Streptomycin:All aminoglycosides have almost similar mechanism of anti-microbial action in 4 steps:1. Attachment of the drug to specific receptor protein ( P 12 in case of streptomycin) on the 30 S subunit of the microbial ribosome.2. Blocks the normal activity of the “ initiation complex” of peptide formation (mRNA + formyl methionine + tRNA)3. The mRNA is misread on the “recognition region” of the ribosome. Wrong amino-acids are inserted into the peptide leading to non-functional protein.4. Break up of polysomes to monosomes (incapable of protein synthesis).These four activities occur simultaneously and are irreversible events leading to killing (bactericida) of the m.o.
Resistance to Aminoglycosides: 1. Lack of receptors on the 30 S ribosome (chromosomal mutants). 2. Enzymes production destroying the drug (plasmid- mediated). 3. Permeability defect in the outer membrane leading to inhibition of the transport of the drug, therefore, can not reach to the ribosome. 4. Natural resistance which chromosomal in nature,e.g., Streptococci are relatively impermeable to the drug. 5. Anaerobic bacteria are resistant because the drug transport is an energy- requiring process that is O-dependent.
Gentamicin: Is bactericidal for many gram-positive and gram-negative bacteria including strains of Proteus, Serratia, & Pseudomonas. It is ineffective against streptococci & bacteroides. It is ototoxic & nephrotoxic (as other aminoglycosides). Aminoglycosides are active in alkaline pH. It can be used with carbenicillin in the treatment of Pseudomonas spesis in emergencies. Also can be used topically as cream or ointment. Tobramycin: It closely resembles gantamycin and there is some cross-resistance between them. Less nephrotoxic than gentamicin. Amikacin: It is a semisynthetic derivative of kanamycin. It is relatively resistant to several of the enzymes that inactivate gentamycin & tobramycin and therefore can be used against resistant m.o. To these drugs.
Netilmicin: It shares many characteristics with gentamicin and tobramycin, but it is not inactivated by some bacteria that are resistant to other drugs. It can be used in immunocompromised patients, and very ill patients at high risk of gram negative sepsis. Spectinomycin: It is an aminocyclitol antibiotic (related to aminoglycosides) for I.M. Administration.Its only use is in the single-dose (2.0 g) treatment of gonorrhea caused by beta-lactamase producer & penicillin-allergic patients. About 5-10% of Gonococci are resistant to this drug.
Tetracyclines: All are readily absorbed from the intestine, the distributed to different tissues, but poorly into CSF. They can be given orally, I.M. & I.V., and excreted in the bile, stool and urine. Types of tetracyclines: 1. Tetracycline hydrochloride. 2. Demelocycline 3. Methacycline 4. Minocycline 5. Deoxycycline 6.Tigecycline
Mechanism of action of tetracyclines: They inhibit protein synthesis by inhibiting the binding of aminoacyl-tRNA to the 30 S unit of bacterial ribosome. Resistance Against Tetracyclines: Under the control of transmissible plasmids. Anti-Bacterial Action of Tetracyclines: They are bacteriostatic broad spectrum drugs. 1. Inhibit the growth of both gram negative & positive bacteria. 2. Are the drugs of choice in the treatment of Rickettsiae, chlamydiae and Mycoplasma pneumoniae. 3. Can be used in the treatment of cholera, shigellosis, gonorrhoea. 4. Can be used in combination with streptomycin in the treatment of brucellosis, Yersinia & Francisella. 5. Minocycline: against Nocardia & eradication of meningococcal carrier state, but induces vestibular damage. 6. Can be used for many months in the treatment of Acne.
Chloramphenicol: It binds to 50 S subunit of bacteria, interfers with the binding of new amino acids to nascent peptide chain by interfering with action peptidyl- transfe-rase. It is bacteriostatic broad spectrum drug. It is produced by Streptomyces venezuelae. Resistance: By chloramphenicol acetyl transferase that destroys the drug (under the plasmid control).
Anti-Bacterial Action of Chloramphenicol: It is a stable drug and rapidly absorbed from the intestine, distributed in tissues including CSF & CNS. Most of the drug is inactivated in the liver and excreted in urine. It can be used: 1. Salmonella typhi and paratyphi. 2. Haemophilus influenzae. 3. Meningococci. 4. Anaerobic bacteria & mixed infections. 5. Severe rickettsial infections. 6. Eye infections (except Chlamydia).
Macrolides and Azalides: They bind to 50 S at 23 S rRNA. They interfer with the formation of initiation complex for petide synthesis or interfere with amino-acyl translocation. The resistance is due to the lack of drug receptor. Erythromycins are of TWO types: 1. Macrolides (Erythromycin & Dirithromycin). 2. Azalides (Azithromycin, Clarithromycin & telithromycin). They are produced by Streptomyces erythreus. They are bacteriostatic against gram positive bacteria mainly and some gram negative bacteria . Examples: pneumococci, streptococci, corynebacterium, mycoplasma, Chlamydia trachomatis, Legionella, Campylobacter jejuni and Neisseria gonorrhoea. The new derivatives have more duration of action as dirithromycin, Azithromycin & Clarithromycin. Clrithromycin can be used in the treatment of H. Pylori in combination with other drugs (see lecture on H. pylori).
Lincomycins: They bind to 50 S ribosome ,and are of two types: 1. Lincomycin 2. Clindamycin (Dalacin-C). These drugs bind to the 50 S ribosome of bacteria and inhibit protein synthesis. The resistance against these drugs in by the lack of the drug receptor (under chromosomal control). They effective mainly against gram positive cocci (bactericidal in high doses, bacteriostatic in low doses). These drugs can be used as alternatives to penicillin or erythromycins. They are effective in case bone infections when caused by gram + cocci as Staph. aureus. Clindamycin is very effective against bacteriodes and other anaerobes. These drugs can be taken orally (acid-stable) & I.V. However, the Clindamycin is the first drug in causing pseudomembranous colitis which is caused by Clostridium difficle superinfection (see ampicillin; treated by Vancomycin).
Fusidic Acid It has a structure similar to that of bile salt. It is active against penicillinase-producing Staph. aureus. Infections such as osteomylitis, endocarditis or septicaemia. Resistance to the drug develops rapidly, so it should be given with other antibiotics. It is absorbed orally, and is relatively expensive drug. It is a safe drug and can be given during pregnancy, however, it may be hepatotoxic.
Oxazolidinones Linzolide: It inhibits protein synthesis by binding to 23S ribosomal RNA. It is active against gram positive bacteria such as vancomycin-resistant Enterococcus faecium, meticillin-resistant Staph. aureus, and penicillin-resistant Strep. pneumoniae. It is useful in different life-threating hospital infections such as bacteraemia, pneumonia or soft tissue infections. It can be given both orally and I.V. It can cause GI disturbances, hypertension, thrombocytopenia, or optic neuropathy.
IV. Anti-Microbial Action Through inhibition of Nucleic Acid Synthesis: Examples: Sulfonamides, Trimethoprim, Trimetrexate, Pyrimethamine, Rifampin, Quinolones, Novobiocin, Anti-viral drugs. Sulfonamides: They are broad spectrum bacteriostatic ( in ordinary doses) and bactericidal (in high doses) drugs. They have a similar structure to a substance called Para Amino Benzoic Acid (PABA), see Fig. Mechanism of Action of Sulfonamides: They interfere with the synthesis of nucleic acid (see below).
Steps of DNA Formation & Sulfonamides: DNA is formed by ATP condensation of a substance called Pteridine which normally binds with PABA through the action of an enzyme called dihydropteroate synthetase to form dihydropteroic acid which then forms dihydrofolic acid. The latter will be converted to tetrahydrofolic acid via the action of an other enzyme called dihydrofolic acid reductase. Then, purines are synthesizedfollowed by the DNA formation. See Fig.
Action of Sulfonamides on DNA Formation: 1. Competition with PABA due to their structural similarity. So, the sulfonamides are inserted instead of PABA in the structure of the formed folic acid rendering it non-functioning. 2. Sulfonamides can inhibit the enzyme dihydropteroate synthetase and this will block the formation of folic acid. Trimethoprim: This drug inhibit the DNA formation by blocking the action of dihydrofolic acid reductase, therefore, inhibits the formation of tetrafolic acid and consequently DNA formation.
Sulfonamide + Trimethoprim (co-trimoxazole): Combination of 5 parts sulfamethoxazole (sulfonamide) and 1 part trimethoprim to form a drug called co-trimoxazole which can block the whole process of DNA formation. This is a type of synergism that is called sequential synergism (blocks the sequences of DNA formation). Pyrimethamine (Daraprim): This drug acts on dihydrofolic acid reductase. It has similar action to trimethoprim. It can be used in the treatment of Toxoplasmosis inparticular with combination with sulfonamides to block the whole DNA formation.
Quinolones:These are synthetic analogs to the original quinolone “Nalidixic Acid”. They inhibit the bacterial DNA synthesis by blocking the DNA gyrase. There are 4 generations (groups) of quinolones. The 2nd, 3rd, & 4th generations are flourinated (contain F). See Fig. These drugs affect gram positive, grame negative and anaerobic bacteria. These effects could be strong, moderate, or weak. See Table.
Anti - Microbial effects of Quinolones: I. First Generation (Nalidixic acid, cinaxacin, Oxalinic acid): They do not have systemic anti-bacterial effects when taken orally, but local effects. Nalidixic Acid (prototype) is a urinary antiseptic acting only on the urinary tract through its excretion and bacteria develop resistance to it rapidly. It may be used in the treatment of malaria and GIT infections in children.
Cont./…Anti - Microbial effects of Quinolones:2. Flourinated quinolones (2nd generation: Ciprofloxacin, Enoxacin, Ofloxacin; 3rd & 4th generations: Cinafloxacin, Gatifloxacin, Levofloxacin, Gemifloxacin) :: They have systemic effects. They are distributed in different tissues except CNS (not used in the treatment of meningitis). Their half-life varies from 3- 8 hours and are broad spectrum drugs ,i.e., can be used in the treatment of different infections:A. Ciprofloxacin is the drug of choice in the treatment of typhoid and paratyphoid fevers (enteric fevers) in a dose of 250-500 mg twice daily.B. Active against bacteria of Enterbacteriaceae Family including those that are resistant to the 3rd generation cephalosporines, Hamophilus species, Neisseria, Chlamydia, and others.
(Cont./… Flourinated quinolones : C. Pseudomonas aeruginosa and Legionella are inhibited by large doses.D. Some are active against multi-drug resistant S. pneumoniae, nafcillin-resistant staphylococci, and E. faecalis E. Newer quinolones are effective against anaerobic bacteria.F. Have activity against Mycobacterium, e.g., M. tuberculosis.
Clinical uses of quinolones: 1. Urinary tract infections including prostatism. 2. Sexaully transmitted diseases caused by N. gonorrhoeae and C. trachomatis, but not T. Pallidum. 3. Lower respiratory tract infections. 4. Enteritis caused by Salmonella, Shigella, or Campylobacter. 5. Major gynaecological and soft tissue infections. 6. Osteomyelitis of gram negative origin. 7. Cystic fibrosis exacerbations caused by Pseudomonas ( about 1/3 are resistant).
Rifampin: Refampin is a semisynthetic derivative of rifamycin, an antibiotic produced by Streptomyces mediterranei. Mechanism of action: It binds strongly to DNA-dependent RNA polymerase and thus inhibits RNA synthesis in bacteria. Anti-Microbial Effects: It can penetrate phagocytic cells well and can kill intracellular organisms. It is active against some gram positive and gram negative cocci, some enteric bacteria, mycobactria, chlamydia, and poxviruses late stage assembly). So, it is a broad spetrum bastriostatic (usual doses) and bactericidal (high doses) drug. Drug resistance can emerge rapidly against it when used alone, therefore, it should be used in combination with other drugs.
Clinical uses of Rifampin: Rifampin is well absorbed after oral administration, widely distributed in tissues, and excreted through the liver (mainly) and urine. It gives an orange colour to the urine, sweat, and contact lenses. Dapsone: Dapsone is a sulfone closely related to sulfonamides. It is used with refampin in the treatment of leprosy.
CLINICAL MICROBIOLOGY OF ANTI - MICROBIAL AGENTS
Measurment of anti-microbial activity:1. Dilution test: Series of dilutions of the drug are done + m.o. To see what dilution can kill or inhibit the m.o. This can be done in test tubes, agar, or microtiter plates.2. Diffusion method (Kirby-Bauer) on solid media as Muller – Hinton agar, or sensitest agar.Factors affecting anti-microbial activity:1. pH of environment (acid or alkaline).2. Compounds of medium.3. Stability of the drug.4. Size of inoculums.5. Length of incubation.6. Metabolic activity of m.o.
Selection of Anti-Microbial Agents: 1. Aetiologic diagnosis: A. Clinical impression B. Best guess which is based on many factors as site of infection, age of patient, place where the infection is acquired, predisposing factors as catheter, I.V., or host factors as immunodeficiency, transplantation or coticosteroid therapy. 2. Anti-Microbial sensitivity test. 3. Serum assay of bactericidal activity.
Dangers of indiscriminate use of anti-microbial drugs: 1. Wide spread of hypersensitivity. 2. Change of body flora leading to superinfections. 3. Masking serious infections without eradicating it as abscesses. 4. Direct drug toxicity, e.g., aplstic anaemia with chloramphenicol, or ototoxicity and nephrotoxicity with aminoglycosides. 5. Drug resistanant m.o. Emerge because sensitive ones are killed or eradicated specially in hospitals.
Anti – Microbial drugs used in combinationIndications:When 2 or more drugs are used together:1. Prompt treatment in desperately ill patients according to “best guess”.2. Delay microbial mutants resistance to the drugs specially in chronic diseases as tuberculosis.3. Treatment of mixed infections as massive trauma.4. To obtain bactericidal synergism in the treatment of some infections as enterococcal sepsis.
Disadvantages of combined drug use: 1. Relaxation of efforts to establish a specific diagnosius. 2. Increase drug reaction or sensitization. 3. The cost of the drugs is high. 4. No more effect than a single therapy is obtained. 5. Drug antagonism may occur (see below). Types of drug combinations: 1. Indifference; not more than the effect one of them. 2. Additions: Sum of actions. 3. Synergism: The combined activity is significantly greater than the sum of both. 4. Antagonism: The effect is less than that of one of them when used alone.
Types of Synergism: 1. Sequential block of a microbial metabolic pathway (sequential synergism); example sulfamethoxazole + trimethoprim (cotrimoxazole). 2. One drug enhances the uptake of the second one; example penicillins enhance uptake of aminoglycosides. 3. One drug may affect the cell membrane and facilitates the entery of the second drug; example: polymyxins + cotimoxazole or rifampin. 4. One drug prevents the inactivation of a second drug by microbial enzymes; example: clavulanic acid protects penicillins from action of beta-lactamases.
Anatagonism:Antagonism is limited by time – dose - relationship. It occurs when bacteriostatic (as chloramphenicol,or tetracycline) drug is given with batericidal drugs (as penicillins or aminoglycosides). This may occur specially when the bacteriostatic drug reachs the area of infection before the bactericidal drug. This should not be done if killing of bacteria is essential for cure ( e.g. tosillitis by Streptococcus pyogenes). To overcome antagonism high doses should be used of one or both drugs.