Microbiology
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Staphylococcaceae Lecture.1
د. انسام محمد
The members of this family are gram positive, catalase positive cocci forming irregular
clusters. They grow aerobically and can be facultative anaerobic. It includes
Staphylococcus and other genera.
Genus staphylococcus
Staphylococci are gram positive spherical cells arranged in single, pairs, tetrads or
usually in grape-like irregular clusters.
Non motile, non spore forming, aerobic or facultative anaerobic and are catalase
positive, oxidase negative and resistant to bacitracin.
The genus staphylococcus is composed of several spp. Many of them are part of the
normal flora.
Staphylococci are initially differentiated by coagulase into:
1-
Coagulase positive S. aureus (pyogenes)
2-
Coagulase negative (CoNS): more than 35 spp. of CoNS is identified those of clinical
significance are:
S. epidermidis (albus)
S. saprophyticus (citreus)
Morphology and identification of Staphylococci:
Microscopical appearance:
Staphylococci are gram positive spherical cells about 1µm in diameter, arranged in
irregular clusters, non motile and non spore forming.
Cultural characteristics
Staphylococci grow on most bacteriological media in aerobic and facultative anaerobic
conditions. They grow rapidly at 37Cº but form pigments best at room temperature.
Colonies on solid media are round smooth raised and pigmented.
Staphylococcus aureus
usually forms golden yellow colonies.
Staphylococcus epidermidis forms white colonies.
Staphylococcus saprophyticus 50% forms lemon yellow colonies.
Staphylococci are resistant to drying, heat (50Cº) for 30 min and are inhibited by certain
chemicals. Staphylococci grow in the presence of 10% NaCl between 18°C and 40°C.
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Microbiology
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Staphylococcus aureus
It is the most important isolate in this family, and is responsible for infections from
mild to a life-threatening infection. It
’s an important cause of hospital acquired infections.
The anterior nares are the principal habitat for S. aureus. Approximately 50% of
people are either permanently or intermittently colonized. Three types of S. aureus nasal
carriers have been distinguished: persistent carriers, intermittent carriers, and non
carriers.
From the anterior nares S. aureus can be transferred to skin and other body areas.
Nasal colonization plays a crucial role as a source of invasive infections. The throat, the
intestine, the vagina, the intertriginous skin folds, the axillae, and the perineum are also
found to be regularly colonized. Increased drug resistance in the last decade is an
important concern.
Staphylococci are also found regularly on clothing, bed linens, and other fomites in human
environment. The pathogenicity of S. aureus is the combined effect of extracellular factors
and toxins together with the invasiveness of the strain.
Antigenic structure
1- Peptidoglycan a polysaccharide provides rigidity to the cell wall and is destroyed
by strong acid or lysozymes.
2- Tiechoic acid linked to peptidoglycan and can be antigenic. It plays an important
role in adherence, activation of complements and chemotaxis (movement of cells
in response to chemical stimuli) of PMN (polymorph nuclear leukocytes).
3- Protein A has the ability to bind to the FC (fragment of crystallization) portion of all
IgG, so it prevents specific antibody from binding to the bacteria.
4- Capsular polysaccharides: its present in some strains and it prevents the
ingestion of the MO by the PMN.
Enzymes and toxins
Staphylococci can produce diseases through their production of many extracellular
substances as enzymes and toxins.
Enzymes
Microbiology
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1- Catalase: produced by all staphylococci.
2- Coagulase: only staphylococcus aureus produces coagulase that binds to
prothrombin and deposit fibrin on the surface of staphylococci and alter their
ingestion by phagocytic cells. There are two types of coagulase
a. Bound coagulase or called clumping factor
b. Free coagulase (staphylocoagulase) it is found free form in the medium and directly
convert fibrinogen to fibrin leading to clot formation.
3- Staphylokinase it causes fibrinolysis.
4- Hyaluronidase.
5- Lipases.
6- Proteinases
7- B- lactamase.
Toxins
1. Exotoxins as Alpha toxin, beta toxin.
2. Enterotoxin: Heat stable exotoxin resist boiling for 30min.causes food poisoning.
3. TSST-1(toxic shock syndrome toxin-1).
4. Exfoliative toxin is epidermolytic toxin causes staphylococcus scalded skin
syndrome leading to generalized desquamation.
5. Leucocidin which kills WBC.
Infections caused by S. aureus:
1-
Superficial infections:
S. aureus infections are suppurative (abscess forming)
a. Impetigo
b. Folliculitis, furuncles (boils), styes
c. Carbuncles
d. Wound infection
2-
Deep infections
a- Osteomylitis
b- Pneumonia
c- Acute endocarditis
d- Arthritis, meningitis, bacteremia, septicemia and abscess of brain and kidney.
3-
Toxin mediated illness
Microbiology
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1.
Scalded skin syndrome (Ritter's disease) it is characterized by rash over most
of the body, like burned skin. Upper layer of skin peel off. It occurs most often in
new born baby with low mortality rate (3%). It also occurs in adults in patients with
severe infections with low immunity so it had a poor prognosis with high mortality
rate (50%). S. aureus produce toxin at the site of infection and the toxin travel to
the blood.
2.
Staphylococcal food poisoning: caused by staphylococcal toxin released in the
food (not ingestion of live bacteria and there is no infection site) so there is no
benefit if we heat the food to prevent infection because the toxin not affected by
heat (heat stable enterotoxin), it causes vomiting within 30min-7h, abdominal pain
with or without diarrhea, no fever and resolve with in 24h.
3.
Toxic shock syndrome: it is fatal multi-systemic disease. Toxic shock syndrome
is manifested by an abrupt onset of high fever, vomiting, diarrhea, myalgia, rash,
skin peeling and hypotension with cardiac and renal failure in the most severe
cases. The mortality rate is 1-3% if treated, if not it may reach70%. There is a strong
association between TSS and tampon use in which the primary site of infection is
the vagina and during menstruation the bacteria can grow on tampon and
accumulate to a dangerous level if not changed. The disease is caused by toxin
released by the bacteria. It often occurs within 5 days after the onset of menses in
young women who use tampons, but it also occurs in children or in men with
staphylococcal wound infections.