Staphylococcus

Molecular and Microbiology

Only susceptible to vancomycin, so very scary

Causes variety of infections from mild skin infections to fulminant toxic shock syndrome or endocarditis

2^nd most common cause of nosocomial infections and endocarditis

coagulase (+) = S. aureus (most pathogenic member)

coagulase (-) = S. epidermidis (IV device infections), S. hemolyticus (peritoneal dialysis infections), S. saprophyticus (UTI)

Staphylococcus aureus

Basics: b -hemolytic; facultative anaerobe; cat. (+); non-motile; clusters; colonies = smooth, round, opaque (golden yellow)

Lab ID: coagulase test: expressed by 97%; slide test: detects clumping factor, distinct from coagulase

Typing: phage typing, plasmid profiles and pulsed field gel electrophoresis (PFGE)

Cell Wall: Peptidoglycan(tightly crossed linked); teichoic acid(phage receptors); Protein A(linked to peptido; interacts with Ig)

Plasmid types: large, conjugative, multi-resist; medium, non-conjugative, multi-resist; small, non-conjugative, multi-resist

Resistance strains: strains found which express resist to every antimicrobial except vancomycin

b -lactamase-mediated penicillin resistance: 95% are now resistant to penicillin; spread occurs via transduction

methicillin-resistant S. aureus (MRSA): mediated by chromosomally-encoded genes which result in production of a penicillin-binding protein (PBP 2a) that binds methicillin with low affinity; not transferable between strains; clonal in nature

Antigenic structures:

ribitol teichoic acid: induces antibody formation and extracellular moieties induce abortive, complement-consuming reactions

Protein A: interacts with Fc portions of IgG

Others: peptidoglycan, clumping factor (binds fibrinogen) and capsular polysaccharide (anti-phagocytic)

Virulence Factors:

capsular polysacch: anti-phagocytic, mediates adhesion to plastic devices, biofilm stabilizes and protects colonies

protein receptors: bind fibronectin and laminin

laminin: major protein in basement memb

Extracellular Enzymes:

Lipases: promote colonization of sebaceous regions (boil formation)

Hyaluronidase: promotes spread thru connective tissue

Coagulase, staphylokinase, nuclease: significance unclear

Extracellular Toxins:

Alpha toxin: causes damage to tissues of the circulatory system, musculature and renal cortex

Beta toxin: (sphingomyelinase) – interaction with RBC membranes

Delta toxin: possesses detergent-like properties

Gamma toxin: significance unclear

Leukocidin: attacks leukocytes and macrophages

Enterotoxins: A, B, C, C₂, D, E

emetic receptor site in abdominal viscera; move to CNS vomiting ctr thru vagus n. and sympath; act as superantigens

Exfoliative toxins: ETA (chromosomal), ETB (plasmid)

phage group II strains; lysis of intracell attachmnt between cells of granular layer of epidermis; dont elicit inflam response

Toxic Shock Syndrome Toxin – 1: (TSST-1)

IL-2 expression, proliferation of human T-lymphocytes, stimulation of IL synthesis by monocytes; acts as superantigen

Epidemiology of S. aureus infection:

normal component of flora; adult carrier rate = 30%; spread is person to person; drug users are more prone
anterior nares = best place to test for carriage; may be eliminated by topical treatment with antibiotic mupirocin
ability to spread MRSA from nasal colonization is enhanced by coexistence of upper respiratory infection
MRSA infection in surgical patients is tied to colonization of a member of surgical team; control problems for hospitals
MRSA carriage may persist for years after initial colonization

Pathogenesis:

penetration of sebaceous glands or hair shaft; immune response controls the organism
higher rates of infection with PMN chemotaxic disorders (Job’s syndrome) or granulocyte killing disorders (chronic granulomatous disease); In hospitals: surgery, burns, decubitus ulcers, IV lines and viral upper respiratory illnesses

Clinical Syndromes:

Furuncles and Carbuncles: most common bact infection; form in 3 days, coalesce, drain; antimicrobials not needed

Impetigo: common in young children; superficial skin infection manifested as encrusted pustules; highly contagious

Pneumonia: preceded by viral prodrome (influenzae); 75% of cases in infants; common with impaired host defense

Osteomyelitis: primarily in young males; exposed bones predisposes; may be complication of trauma; hematogenous spread; occurs at metaphyseal equivalents (vertebral bodies, sacroiliac joints, sterno-clavicular joints); responsible for 50% of septic arthritis

Staph scalded skin syndrome (SSSS): 3 levels produced by infection with strains producing ETA or ETB

Generalized exfoliative dermatitis, painful erythema, bulla formation
Bullous impetigo – localized form of above
Staph scarlet fever – occurs primarily in infants and children

Toxic Shock Syndrome (TSS):

Most common in menstruating women
Fever, decreased blood pressure, diarrhea
myalgia, rash
absence of antibody may predispose

Food Poisoning: Most common form in US; fever uncommon

Endocarditis:

Common in IV drug users – involves right heart valve
Can lead to metastatic infection in diverse locations (bone, spleen, psoas muscle, joints)
Commonly acquired in hospitals; Requires bactericidal antimicrobial therapy

Cellulitis: infection of subcutaneous tissue; common in lower extremities of diabetics and patients with venous or lymphatic obstruction

Pyomyositis: infection of skeletal muscles; more common in tropical than temperate climates

Bacteremia: most common cause of nosocomial bacteremia; frequently related to indwelling vascular devices

Diagnosis: gram stain of purulent secretions; culture; teichoic acid antibody

Treatment:

drain infections

b -lactamase-stable penicillin, cephalosporin, clindamycin, imipenem; vancomycin only reliable treatment for MRSA

b -lactamase is produced by bug Þ kills penicillin

95% resistant to penicillin/ampicillin; strains exist with resistance to everything but vancomycin

Staphylococcus epidermidis

coagulase (-); low pathogenicity

2^nd most common cause of nosocomial infection due to its ubiquitous colonization of human skin, proficiency at colonizing prosthetic devices (IV devices and lines) and its resistance to many common antimicrobials

binds efficiently to plastic material thru production of polysacch (slime) which also stabilizes and protects the colonies

majority are resistant to methicillin, leaving only vancomycin as effective treatment

Staphylococcus saprophyticus

coagulase (-); cause UTIs; primarily in patients > 50 yo

generally susceptible to most urinary tract antimicrobials with exception to nalidixic acid