Gram-positive cocci (0.7 - 1.2 µm),
usually in clusters (Gr. staphyle, bunch of grapes)
"tissue" invasive, produce purulent (pus-filled) lesions
extracellular pathogens (destroyed within leukocytes)
peptidoglycan distinguished by the pentaglycine cross-bridge.
colonies are often surrounded by a clear zone of hemolysis (beta hemolysis) due to production of hemolysins; species specificity
withstand heat at 60C for 30 min
grow in media containing 7.5% to 10% NaCl
remain viable for months on agar plates stored at 4C
grows at 6.5°C to 46°C
grows at pH 4.2 - 9.3
reside on mucous membranes and skin of mammals (60% intermittent carriers)
anterior nares and on mucous membranes of about 30% of the population
carriage rate in considerably higher among hospital personnel
source of infection - individual's own microflora (or from hospital staff)
can survive on inanimate objects (lab coats, gloves, bedding)
S. aureus = coagulase-positive staphylococci of human origin
Examples of coagulase-negative staphylococci: S. epidermidis, S. saprophyticus
important hospital and community pathogen
most common cause of bacterial infection in humans
often the result of hospital-acquired infections
S. aureus causes a wide variety of suppurative diseases in humans:
Cutaneous lesions (superficial abscesses)
folliculitis - infection of hair follicle
boil or furuncle - extension to subcutaneous tissues
impetigo - skin infection (strep or staph)
scalded skin syndrome
persons with traumatic or operative wounds
burn victims or other serious skin lesions
chronic debilitating disorders (diabetes mellitus, cancer, cystic fibrosis)
IV drug abusers
metastatic infection - deep abscesses
infections of various internal organs
endocarditis, meningitis, pneumonia, cystitis, septicemia, osteomyelitis (inflammation of the bone)
infections of prosthetic devices
toxic shock syndrome
S. aureus is the etiologic agent of several toxin-mediated diseases: food poisoning, scalded skin syndrome, and toxic shock syndrome.
animals - mastitis
-important in determining relatedness of outbreak strains (single source?) --source of outbreak if outbreak strains are related (GP: no O, H, K antigens)
a. phage typing (sensitivity to a standardized set of bacteriophages)
b. serotyping: based on variability of surface antigens, e.g., teichoic acid, capsule (not useful). Serotyping of S. aureus not useful - lack of variable antigens that are serologically identifiable.
c. antibiotic resistance patterns
d. plasmid analysis
e. chromosomal genes or transposons analyzed by Southern blot hybridization
**f. pulsed-field gel electrophoresis (rare-cutting restriction enzymes)
Staphylococcal diseases are multifactorial (not a single virulence factor)
A. Surface antigens
1. Capsular polysaccharides. Eleven serotypes have been reported. Types 1 and 2 are highly encapsulated, mucoid strains that are virulent for experimental animals, but rarely encountered among clinical isolates of S. aureus. Serotypes 5 and 8 account for ~75% of all strains. These strains produce a "microcapsule" which may be antiphagocytic. Not clear if antibodies are protective.
2. Teichoic acids (PG linked); lipoteichoic acids (membrane associated). Regulates cationic environment.
3. Protein A. 40-60 kD surface protein (linked to PG) of virtually all strains of S. aureus; binds the Fc portion of IgG. May interfere with phagocytosis, because of steric inhibition of Ab binding, and competition with phagocytic cells for the Fc portion of specific opsonic antibodies.
4. Adhesins. Specific CW associated surface proteins of S. aureus that bind to matrix proteins such as fibronectin, fibrinogen (clumping factor), collagen, bone sialoprotein, etc. These binding activities are thought to be involved in binding and colonization of various body sites and specific cell types.
B. Extracellular proteins
*a. Toxins (hemolysins) - cytolytic, membrane-damaging toxins
• alpha toxin (hemolysin) - 34 kD protein; principal hemolysin expressed by human strains of S. aureus. Toxin monomer binds to cell surface -> heptamer formation -> transmembrane channels. Hydrophobic molecule that inserts within the cell membrane of susceptible cells, forming pores.
Most active against rabbit erythrocytes, but also active against human platelets. Inject alpha toxin IV -> hemolysis, dermonecrosis, lethality
In experimental infections with S. aureus strains - the severity of the infection was affected by a-toxin expression:
• beta toxin (hemolysin) produced commonly by animal strains of S. aureus and only by 10 - 20% of human isolates.
beta toxin is a sphingomyelinase C enzyme - catalyzes hydrolysis of membrane phospholipids.
• gamma toxin (hemolysin). Mode of action - unknown.
• delta toxin (hemolysin) is produced by most human strains of S. aureus. 10,000 to 100,000X less toxic than alpha toxin. Has detergent-like properties.
b. Pyrogenic (fever-inducing) exotoxins - superantigens.
Single-chain, 25- to 30-kDa proteins.
Superantigens (Sags) bind directly to major histocompatibility complex (MHC) molecules outside of the antigen groove without needing to be processed by antigen presenting cells for presentation to T cells. Massive proliferation of T cells that bear specific Vb elements in their antigen receptors leads to overproduction and/or release of cytokines, causing clinical symptoms that include fever, hypotension, and shock (typically seen in toxic shock syndrome).
Sag expression can result in acute systemic disease in humans. This is intimately linked to high serum levels of interleukin –1 (IL-1), -2, and –6, interferon gamma, and tumor necrosis factor (TNF) released by activated lymphocytes.
Thus Sags activate multiple compartments of the host immune system. They are potent T-cell mitogens (induce nonspecific T-cell proliferation).
The frequency of T cells that respond to a conventional ag is low (1 in 104 to 106 T cells). The percentage of T cells responding to a particular Sag can be very high (5 to 25%).
• staphylococcal enterotoxins are superantigens. They induce emesis and diarrhea – act by stimulating the vagus nerve endings in the stomach lining that control the emetic response. Nausea, cramps, vomiting and diarrhea appear abruptly 2 to 8 h after ingestion of contaminated food. Recovery generally occurs within 24 h. Not treated; self-limiting. Several distinct serotypes of the enterotoxins exist: A –> E, G, and H.
Food (commonly those that are high in protein: ham, poultry, potato and egg salads, pastries, custards, and salad dressing)
usually contaminated by food handlers that are carriers of S. aureus
Enterotoxin-mediated food poisoning:
occurs when a contaminated food is held at inappropriate temperatures long enough to allow the bacteria to elaborate toxin
heat-stable toxins (withstand 100C for 30 min)
toxins not inactivated by digestive enzymes
food must be refrigerated before as well as after cooking
toxin-contaminated food has normal odor, appearance, taste
Systemic infection with an SE-producing strain - more severe than strain that is enterotoxin negative.
• toxic shock syndrome toxin-1 (TSST-1)
22 kDa protein
causes toxic shock syndrome - a multisystem, febrile illness with abrupt onset and shock
local infection with TSST-1-producing organisms
TSST-1 not directly toxic to cells; causes over-stimulation of T cells with outpouring of lymphokines/cytokines
TSS is characterized by shock, fever, hypotension, multiple organ system failure, nausea, vomiting, diarrhea, hyperemia of mucous membranes, and a rash followed by desquamation (palms and soles). Mortality ~2%.
most cases occur in association with menstruation with certain (high absorbency) tampons playing an important accessory role
~20% of cases occur in association with postoperative and other local S. aureus infections at various sites. S. aureus implicated in nonmenstrual TSS may produce TSST-1 or one of the other enterotoxins, most commonly SEB.
Most individuals have protective levels of antibody to TSST-1. Those who get TSS lack anti-TSST-1 antibodies.
30-50% of S. aureus strains produce one or more enterotoxins; ~15% of S. aureus strains produce TSST-1.
two forms of toxin - A (chrom gene) and B (plasmid gene)
toxin cleaves the stratum granulosum of the epidermis; causes lysis of the intracellular attachment between cells of granular layer of epidermis.
causes exfoliative dermatitis - blistering, loss of epidermis
not fatal - condition is limited largely to the superficial layers of the skin; no cell death.
produced by ~5% of S. aureus strains and affects babies, not adults.
Other staphylococcal exoproteins – not superantigens:
c. leukocidin - toxin that acts on human PMNs and macrophages. Produces an ion channel in the membrane
two protein components (S and F) that act synergistically -> cytolysis
d. coagulase - hallmark protein of S. aureus strains; a plasma-clotting protein. It binds to prothrombin - to convert fibrinogen to fibrin. Different from normal mammalian clotting mechanism. ?role in virulence?
e. Staphylokinase (fibrinolysin): dissolves fibrin clots by promoting the conversion of plasminogen to the fibrinolytic enzyme plasmin. Phage-encoded gene (phage conversion)
f. Nucleases - act on extracellular DNA and RNA.
g. Lipases - promote survival on skin. Activity on cell membranes?
h. Hyaluronidase - spreading factor; hydrolyzes hyaluronic acid present in the intracellular ground substance of connective tissue.
i. Proteases - at least 3.
antibiotics 4-6 weeks for serious infections
drain abscesses; remove foreign bodies
resistance to methicillin and vancomycin - emerging problems
Coagulase-negative staphylococci (CoNS)
~11 species that infect humans
Major spp of CoNS are S. epidermidis and S. saprophyticus.
normal skin flora
hospital-acquired infections; from patient skin or skin of hospital personnel.
infections of prosthetic devices (artificial heart valves, CNS shunts, hip prostheses, other orthopedic devices), indwelling catheters, vascular grafts, peritoneal dialysis, and wounds
produce slime (biofilm) - secreted material - TA, DNA, other polysaccharides
facilitates bacterial adhesion
acts as a barrier to antibiotic penetration
usually resistant to multiple antibiotics, including Me and Pen.
agent of UTI, particularly in sexually-active young women.
novobiocin resistant which distinguishes it from S. epidermidis
Other coagulase-negative species are important as animal pathogens.