Microbiology
Notes…
1
Staphylococcaceae Lecture.2
د. انسام محمد
Coagulase negative staphylococci:
Overall, the clinical appearance of infections caused by CoNS has been characterized as
associated with subacute or chronic clinical courses without fulminant signs, and they are
seldom life-threatening.
a.
Staphylococcus epidermidis:
S. epidermidis is a member of the normal flora of human skin, respiratory and
gastrointestinal tracts. S. epidermidis is a nonpathogenic, noninvasive coagulase
negative novobiocin sensitive and tend to be non-hemolytic.
The role of this MO as an etiologic agent of diseases has become increasingly evident.
This MO is always isolated from the normal flora of skin. Infection is predominantly
hospital acquired. Such organism rarely produces suppuration but may infect orthopedic
or cardiovascular prostheses or cause disease in immune-compromised persons.
The MO forms on catheters Biofilm layer which is the key component in bacterial
held together by a mucus like matrix
that adheres to a surface. Biofilms can form on the surfaces of liquids,
solids, and living tissues. Most antibiotics do not work against them.
Most common hospital acquired infection caused by S. epidermidis are UTIs, prosthetic
valve endocarditis and intravascular catheter and CSF shunt infection. Septicemia
reported in immuncompromised patient.
The virulence factor is the production of the biofilm or slime which is antiphagocytic.
b.
Staphylococcus saprophyticus:
Staphylococcus saprophyticus is non-hemolytic, novobiocin resistant, it cause urinary
tract infection in young women.
It is the second commonest cause of UTI in young female (true urinary pathogen) after
E.coli. It is rarely found on other mucous membrane or skin surfaces. In urine culture it
may be found in low concentration but still considered pathogenic.
S. saprophyticus adheres more than other bacteria to the epithelial cells lining urogenital
tract.
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Microbiology
Notes…
2
Laboratory diagnosis of Staphylococci:
1-
Sample
is taken from the site after proper cleansing of the area and transport
immediately without delay to avoid dryness of the sample.
2-
Microscopical examination:
Direct gram stain smear to see gram positive or
variable cocci arranged in single, pairs tetrad or clusters inside and outside the
PMN.
3-
Culture:
1. Staphylococci grow readily on routine culture medium particularly 5% SBA
(sheep blood agar) within 24 hr. On SBA after 18-24 hr of incubation at 35-37
C° Staphylococcus aureus produce round smooth colony with hemolytic zone
with golden yellow pigment.
S. epidermidis produce non hemolytic white colony.
S. saprophyticus produce larger colonies only 50% produce lemon yellow colonies.
2. Mannitol salt agar (MSA) is good selective and differential culture medium for
isolation of S. aureus. It is diagnosed by mannitol fermentation.
4-
Biochemical reactions:
1. Catalase differentiated Staphylococci from Streptococci.
2. Coagulase: identification of Staph aureus :
a. Slide coagulase: Clumping factor formerly referred to as cell bound
coagulase it causes agglutination in human and rabbit serum detected by
slide method.
b. Tube coagulase: strain of Staph aureus does not produce clumping factor
should be investigated for stapyhlocoagulase production by tube method.
Isolates that do not produce clumping factor or staphylocoagulase are reported as CoNS.
5-
Antimicrobial susceptibility :
Broth micro-dilution or disk diffusion susceptibility testing should be done routinely on
Staph aureus isolates from clinically significant infections.
Resistance to penicillin G
can be predicted by a positive test for β-lactamase;
approximately 90% of S. aureus produ
ce β- lactamase.
Resistance to nafcillin (and oxacillin and methicillin) occurs in about 35% of S aureus
and approximately 75% of S epidermidis isolates. Most staphylococci are capable of
acquiring and using one or more of the resistance mechanisms.
Microbiology
Notes…
3
Methicillin resistant Staphylococci:
Before the introduction of penicillins, invasive S. aureus were fatal in 90% of cases. With
the increased drug resistance of S. aureus we may return to these days. Penicillin
introduced in 1940s and resistance to it was wide spread by 1950s.
The bacteria rapidly acquire B-lactamase which is an enzyme capable of cleaving or
cutting up the drug (penicillins) making it harmless to the bacteria.
Today more than 95% of patients with S.aureus are not cured by penicillins. To combat
this problem, drug companies made methicillin in 1959. Oxacillin and other penicillinase-
resistant penicillin such as methicillin, naficillin, cloxacillin, constitutes the drug class
of choice for treating staphylococcal infection.
Strains resistant to the new drug began to appear within 2 years. The methicillin resistant
strains were also resistant to penicillin and other antibiotics. Today some hospitals report
more than 80% of isolated S. aureus strains are MRSA.
MRSA are now endemic (always present) at many large hospitals (hospital acquired
MRSA). MRSA is extremely hard to control as it has been found in beddings, curtains,
windows and can survive a long time.
The important reservoir of MRSA is the hospital staff, as hospital workers can become
colonized with the drug resistant S. aureus on their skin and nose with no symptoms.
They pass the bacteria to their patients.
Penicillin work by binding to the PBP which is important in the cell wall synthesis, so the
drug blocks cell wall synthesis. MRSA make a new PBP2a (is encoded by mec A gene
are referred to as MRSA) which can make cell wall but no longer bind to antibiotics.
In the past MRSA was associated with hospital acquired infection. Since the late 1990s,
however, MRSA has been found to be associated with community acquired infections
(community acquired MRSA). It occurs more in prisons, military, among athletes.
Patients allergic to penicillin or have MRSA are treated with vancomycin, but increased
resistance to glycopeptides call for restrictive use of these drugs.
Vancomycin resistant staphylococci:
Vancomycin is the drug of choice and sometimes the only drug available for serious
staphylococcus infection, and thus the development of vancomycin resistance has been
a serious concern (sensitive to daptomycin and linezolid).
The most accurate susceptibility is done by non automated method. Detection of these
strains should be reported to the CDC.
Microbiology
Notes…
4
Linezolid:
It is used to treat new strains of vancomycin resistant S. aureus. It is a member of new
class of antibiotics that interfere with bacterial protein synthesis.
Treatment of Staphylococcal infection:
* Minor skin infections are usually treated with an antibiotic ointment. In some cases, oral
antibiotics may be given for skin infections. Additionally, if abscesses are present, they
should be surgically drained.
* More serious and life-threatening infections are treated with intravenous antibiotics.
* Several different types of antibiotics have been used to treat staph infections. The choice
of antibiotic depends on the type and severity of the infection as well as drug-resistance
patterns of the particular bacterial type. Some of the antibiotics that have been used to
treat staph infections are cephalosporines, naficillin,oxacillin, rifampin and clindamycin.
Combinations of antibiotics and other antibiotics can also be used.
* MRSA treated with vancomycin.
* Treatment of vancomycin resistant staph is by the use of daptomycin
In hospitals, areas at high risk for infection are the newborn surgery, ICUs, operating
rooms and cancer chemotherapy. The persons with active Staphylococcal lesions and
carriers may have excluded from these areas and application of topical antiseptics to
nasal or carriage site. Rifampicin coupled with a second oral anti Staphylococcal drug
sometime provide long term suppression and possibly cure of nasal carriage.
Genus Micrococcus
This genus belongs to the Micrococcaceae family consisting of Gram-positive cocci. They
are found free living in the environment and they resemble staphylococci. They form
packets of four or eight cocci.
They are all gram positive cocci. Their colonies can be red, yellow or orange. They are
saprophytic MO, catalase positive, positive for modified oxidase test and sensitive
to Bacitracin.