1
True bacteria – Cocci- Gram positive cocci
Staphylococci
The genus Staphylococcus has at least 35 species:
(A)The coagulase-positive staphylococci: as Staphylococcus aureus , the most
common causes of bacterial infections ,food poisoning and toxic shoke syndrome.
(B)The coagulase-negative staphylococci: as Staphylococcus epidermidis, and
Staphylococcus saprophyticu,are normal human flora and sometimes cause infection.
(A)- Coagulase-positive staphylococci:-
Staphylococcus
aureas:
Epidemiology
It's carried by healthy individuals on the skin and mucous membranes, serves as a
source of infection to themselves and others (direct contact, contamination of food).
Pathogenesis
Staph. aureus produces many enzymes + toxins
+
cell wall virulence factors
which are responsible for pathogenisty(figure1).So the pathogenesis depend on the
combined actions of several virulence factors and it
ś difficult to determine the role of
any given factor.
Enzymes
1. catalase →E. that convert hydrogen peroxide
(H
2
O
2
) to water and oxygen.
2. coagulase
)
→ invasive pathogenic potential factor) converts fibrinogen→fibrin in
plasma causes clot preventing the bacterial cell from being killed by the immune
cells.
3. hyaluronidase
)
→
spreading factor) degrades hyaluronic acid in connective tissue.
4.staphylokinase→E .
causes fibrinolysis by degrading fibrin, and facilitates in
spreading
of bacteria cells.
5. β-lactamase.
6. proteinase.
7. lipase.
8. nuclease.
2
Figure:(1)virulence factors that may play a role in the pathogenesis of staphylococcal infections.
Toxins
Staph. aureus produces exotoxins:-
(1)- cytolytic exotoxins or haemolysin→ toxin that lyses the R.B.Cs as:
α-haemolysin = partially (in completely) lyses R.B.Cs.
β-haemolysin = lyses R.B.Cs completely.
ȣ-haemolysin = lyses R.B.Cs of animals & some times human.
δ-haemolysin=causes distribution of biologic membrane which results in diarrhea.
(2)- leucocidin → lyses the W.B.Cs.
(3)- superantigen exotoxins → toxins have an affinity for the T cell receptor MHC
class П antigen complex. They stimulate an enhanced T lymphocyte response . T cell
activation can cause toxic shock syndrome by release large amounts of T cell
cytokines, such as interleukin-2(IL2), interferon-ȣ(IFNȣ) and Tumor necrosis factor-
α(TNF-α). Superantigen exotoxins as:
(A)- enterotoxins:-
at least six (A-F) soluble toxins produced by 50% of Staph. aureus, differ in
chains of a.a.s. and responsible for gastroenteritis, food poisoning. They cause
vomiting and diarrhea after (1-8 hour).Enterotoxins are heat stable, resist boiling for
30min,and resistance to the action of gut enzyme.
(B)- Exfoliative Toxin (ET)(epidermolytic toxin):-
Causes Staphylococcal Scalded Skin Syndrome (SSSS)occurs in young children.
(C) - Toxic Shock Syndrome Toxin-1 (TSST-1):-
This toxin is structurally similar to enterotoxins (B,C,F).This toxin is associated
with fever, shock, rash ,desquamative of the skin. (TSST-1) is the classic cause of
toxic shock syndrome, because of similarities in molecular structure.This toxin does
not cause food poisoning when ingested.
3
Cell wall virulence factors
(a)- protein A:-
Is a major component of the Staph. aureus cell well, it binds to the Fc of IgG
therefore strongly antiphagocytic effect.
(b)- fibronectin-binding protein (FnBP):-
Staphylococcal surface proteins promote binding to mucosal cells and tissue
matrixes.
Clinical significance
S. aureus causes disease by infecting tissues and /or by producing toxins (figure2)
A common entry to the body:-
(A)-is a break in the skin.
(B)-is the respiratory tract (as a complication of influenza).
(C)-is a digested contaminated food.
The localized host response to Staph. infection is inflammation ,creates a "abscess".
Serious consequences of Staph. infections occur when the bacteria invade the blood
stream, resulting septicemia (the presence and persistence of pathogenic
microorganisms or their toxins in the blood) may be fatal. Bacteremia (the presence
of viable bacteria circulating in the blood stream)may result in seeding internal
abscesses, skin lesions, or infections in the lung, kidney, heart, skeletal muscle or
meninges.
(1)Localized skin infections: -
→ folliculitis = the most common Staph. aureus infections, superficial abscess
involving hair follicles.
→ lmpetigo = a localized superficial spreading crusty skin lesion seen in children (by
staph. aureus or strep. pyogenes or both).
→ furunclosis = subcutaneous abscesses form around foreign bodies (local therapy as
removal of the foreign body, soaking, drainage).
→ carbunclosis = larger deeper skin Infections lead to bacteremia (antibiotic
therapy).
(2)- deep, localized infections:-
These may be metastatic from superficial infections or skin carriage, or may result
from trauma.
→ acute and chronic infection of the bone marrow.
→ acute infection of joint space in children (treated by drainage + an antibiotic).
(3)- acute endocarditis:-
associated with intravenous drug abuse, caused by:-
(A)-injected of contaminated preparations.
(B)-needles contaminated with staph. aureus .
(C)-skin around the injection site which is contaminated with staph. aureus
,which can be introduced into soft tissue and the blood stream, even when a sterilized
needle is used.
4
(4)- septicema:-
Is a generalized infection with sepsis or bacteremia, itś fatal.
(5)- pneumonia:-
Causes of severe, necrotizing pneumonia.
(6)- nosocomial infections:-
Its one of the most common causes of hospital-acquired infections often of
wounds (surgical) or bacteremia associated with catheters. Progression to
septicemia .
(7)- toxinoses:-
a)-toxic shock syndrome (TSST-1);-
Results in high fever, rash, vomiting, diarrhea, hypotension and multiorgan
involvement (especially GI., renal, and /or hepatic damage)by TSST-1 positive
strains of staph. aureus resulting in entry of the toxin into the circulation in the
absence of true infection.
(b)-staphylococcal gastroenteritis (food poisoning):-
Is caused by ingestion of food contaminated with enterotoxin producing Staph.
aureus (such as proteins-rich egg salad, cream pastry refrigerated).Symptoms =
nausea, vomiting and diarrhea are acute following a short incubation period (<6h
s
)
and are caused by local actions of the toxin on GI tract rather than from infection.
(C)- scalded skin syndrome (ssss):-
Resulting from the action of an exofoliative toxin that attacks the intercellular
adhesive of the stratum granulosum causing epithelial desquamation.
(8)- other pyogenic infections:-
→ cystitis =inflammation in urinary bladder.
→ cellulites = occurs in skin and soft tissue.
→ osteomyelitis = occurs in the bone.
→ bronchopheumonia = usually after influenza.
→lung abscess.
→post operative wound infections.
→arthritis.
5
Figure:(2)Diseases caused by staph.aureus
Laboratory identification
Specimens:- as surface swab, pus, blood, tracheal aspirate or spinal fluid.
Microscopic examination
:-
G
+
cocci, spherical cells
staining darkly round cocci
occur like grapes cluster , non motile, non spore forming and non capsulated.
Macroscopic examination (culture and biochemical tests):- Its facultative
anaerobic grow readily on most bacteriologic media at 37 °C, but form
endopigment(from golden-yellow to cream in color)under anaerobic condition at
room temperature (20–25 °C). Colonies on solid media are round, smooth, opaque,
shining surfaces and low convex. Its resists drying, heat (50°C for 30 minutes), and
9% sodium chloride. On the blood agar, colonies lend to be yellow, hemolytic rather
than gray and non haemolytic in coagulase –ve Staph. On selective and differential
media called mannitol salt agar( which includes selective agents are mannitol + 7.5%
Nacl) produces acid that lower ph when ferments mannitol sugar, then the ph
indicator turns the color from pink to yellow, but others types of Staph. spp. will
increase the ph turning the color from pink to deep pink.
Biochemical reactions → coagulase +ve while other S. spp. coagulase – ve,
catalase +ve (which differentiates them from the catalase – Streptococci) , Mannitol +
ve .
Serological and typing tests:- Abs to teichoic acid can be detected in prolonged
deep infections, like osteomylelitis and arthritis. Phage typing is used in severe out
breaks of Staph. aureus infection as might occur in hospital.
6
Treatment
Require aggressive treatment including incision and drainage of localized lesions as
well as systemic antibiotics. Choice of antibiotics is complicated by the frequent
presence of acquired antibiotic resistance determinants.
All community and hospital-acquired Staph. aureus infections are now resistant to
penicillin G due pencillinase-encoding plasmids or transposons.This has required the
replacement of penicillin G, by β-lactamase-resistance penicillins such as methicillin
or oxacillin. Increase use of methicillin and related antibiotics has resulted in
methicillin-resistant- Staph. aureus.
(1)- hospital-acquire methicillin-resistant- Staph. aureus (MRSA):
A high percentage (50%) of hospital Staph. aureus isolates has been found to be
resistant to methicillin or oxacillin. Antibiotics resistance is caused by chromosomal
acquisition of the gene for a nodified pencillin-binding protein (PBP),(PBP-2a),which
codes for new peptidoglycan transpeptidase with a low affinity for all currently
available β-lactam antibiotics,and thus renders infections with methicillin-resistant S.
aureus unresponsive to β-lactam therapy.
Compared with methicillin-sensitive Staph. aureus, MRSA infections are
associated with longer hospital and ICU stays,longer durations of mechanical
ventilation and higher mortality rates.
MRSA strains are also resistant to many other antibiotics, some being sensitive
only to glycopeptides such as vancomycin.
(2)- community acquired MRSA (CA-MRSA).
The most common clinical manifestations of CA-MRSA are skin and soft tissue
infections (abscesses or cellulitis). Less commonly, can cause severe disease such as
necrotizing pneumonia, posteomyelitis, septicemia.
CA-MRSA distinguish from hospital-associated MRSA:
(1)
has DNA fragments
obtained upon enzymic cleavage and electrophoresis,
(2)
it produce specific toxin,
(3)
unique antibiotic resistance pattern,
(4)
sensitive to many antibiotics that do not show
much activity against hospital-associated MRSA include ciprofloxacin, clindamycin ,
with some CA-MRSA sensitive to erythromycin, gentamicin,
Rifampin,
tetracycline,
and trimethoprim-sulfamethoxazole.
(3)- Vancomycin resistance:-
Vancomycin has been the great of choice for treatment of life threatening Staph.
aureus infections. In 1997 several MRSA when isolated had acquired low-level
vancomycin resistance, and itś increased prompting the use of alternative drugs, such
as quinupristin dalfopristin, linezolid, daptomycin. So, Clinical strains of Staph.
aureus are resistant to benzyl penicillin due to the production of pencillinase, a β-
lactamase. Penicillin and amoxicillin are susceptible to S. β-lactamase. Combination
of amoxicillin and clavulanic acid is used in therapy.
Prevention
There is no effective vaccine against Staph. aureus. Disinfection of hands and
fomites. are need in nosocomial Staph. aureus epidemics.
7
(B)- Coagulase-negative staphylococci:-
S. epidermidis. and S. saprophyticus both of them are important agents of hospital-
acquired infections.
Staphylococcus epidermidis:
Present in large numbers of the normal flora of the skin, recovered from blood
contaminant from skin. Despite itś low virulence, itś cause infection of heart valves
and catheters. Acquired drug resistance by S. epidermidis is more frequent than by
Staph. aureus. Vancomycin sensitivity remains the rule (but also reported
vancomycin-resistant isolates). S.epidermidis produced slime that facilitates
adherence to intravenous catheters acts as barrier antimicrobial agents. Its sensitive to
novobiocin
Staphylococcus saprophyticus:
Cause cystitis in women, probably related to its occurrence as a part of normal
vaginal flora. Sensitive to penicillin G. its can be distinguished from S. epidermidis
and most coagulase –ve staph. by its resistance to novobiocin.