Staphylococci

 

Staphylococcus aureus

General characteristics

Gram-positive cocci (0.7 - 1.2 m),

usually in clusters (Gr. staphyle, bunch of grapes)

pyogenic (pus-eliciting)

"tissue" invasive, produce purulent (pus-filled) lesions

extracellular pathogens (destroyed within leukocytes)

nonmotile

nonsporeforming

acute diseases

facultatively anaerobic

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

Hardy microbes

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 6.5C to 46C

grows at pH 4.2 - 9.3

 

Habitat

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)

 

Classification

S. aureus = coagulase-positive staphylococci of human origin

Examples of coagulase-negative staphylococci: S. epidermidis, S. saprophyticus

 

Clinical significance

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

newborns

persons with traumatic or operative wounds

burn victims or other serious skin lesions

chronic debilitating disorders (diabetes mellitus, cancer, cystic fibrosis)

IV drug abusers

wound infections

bacteremia

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

Biotyping:

-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)

g. ribotyping

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

superantigen

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)

superantigen

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.

 

Treatment:

antibiotics 4-6 weeks for serious infections

drain abscesses; remove foreign bodies

resistance to methicillin and vancomycin - emerging problems

Coagulase-negative staphylococci (CoNS)

≥20 species

~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.

nosocomial bacteremia

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

antiphagocytic

usually resistant to multiple antibiotics, including Me and Pen.

S. saprophyticus

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.