Staphylococci Flashcards
Staphylococcus
catalase-positive spherical cocci often appear in grape-like clusters in stained smears
grow well on peptide-containing nutrient medium under aerobic and anaerobic conditions
Staph aureus differentiation
S.aureus is differentiated from others by its production of coagulates, which are capable of clotting blood - use latex agglutination to detect clumping factor and protein A
S. aureus can also be differentiated using FISH and PCR. many of these can detect methicillin-resistant S. aureus
Staph aureus disease
S. aureus may be present among indigenous flora of the skin, eye, upper respiratory tract, GI, urethra, and infrequently vagina
breaks in skin and mucosa, foreign bodies or implants, prior viral disease, and prior antimicrobial therapy, immunodeficiency lead to infection
impetigo, folliculitis, mastitis, surgical wounds
leading cause of bacteremia in hospitalized patients
may cause carditis, particular in left-sided valvular heart disease and injection drug users
most common cause of spinal epidural abscess and suppurative intracranial phlebitis
responsible for many cases of osteomyelitis, and most common cause of septic arthritis in prepubertal children
infrequent cause of community-acuired pneumonia, but common cause of nosocomial
Staph aureus virulence
if capsule is present has antiphagocytic properties
cell wall peptidoglycans have endotoxin-like activity, stimulating the release of cytokines by macrophages, activation of complement, and aggregation of platelets.
Protein A is an immunologically active substance in the cell wall, and has antiphagocytic properties that are based on its ability to bind the Fc fragment of immunoglobulin
other surface molecules recognize adhesive matrix molecules, enhancing ability to colonize
Produces many toxins:
exotoxin TSST-1 is responsible for toxic shock syndrome
enterotoxins A-E are responsible for food poisoning 1 - 6 hours after ingestion
epidermolytic toxins A and B cause skin erythema and separation of scalded skin syndrome
also produce several enzymes, including protease, lipase, and hyaluronidase which destroy tissue
CA-MRSA
Community-acquired methacillin resistance staph aureus
more common last 10 years
Panton-Valentine leukocidin toxin (PVL), which is rarely associated with hospital-acquired strains
responsible for necrotizing skin and soft tissue infections and rarely fatal pneumonia
mostly children involved in contact sports and those living in institutions like prisons
unlike HA-MRSA, these are often susceptible to non-B-lactam classes of antibiotics
HA-MRSA
Hospital-acquired methicillan resistant staph aureus
usually resistant to all antibiotics except the glycopeptides, such as vancomycin
- mecA gene is responsible for penicillin-binding protein
CNS - coagulase negative species S. aureus
associated with foreign bodies, especially implanted prosthetic valves, joints, and shunts.
more than 20 species
S. epidermidis most common
S. saprophyticus most important cause of bacteriuria, particularly among sexually active young women
S. hemolyticus is rarely isolated, but resistant to vancomycin unlike most CNS
Staphylococcus Laboratory Diagnosis
observation of typical rounded, gram-positive cocci in clusters
S. aureus produces coagulase causes plasma to clot
95% of S. aureus identified by slide cogulase test, 100% by tube coagulase test
slide test - clumping occurs within 30 secods
S. lugdunensis and S. schleiferi may also give positive slide test.
run a control with emulsified colony to make sure auto agglutination does not occur
tube coagulase test is incubated for four hours, then again for 24 hours if negative. some strains produce fibrinolysin which can lyse the clot after four hours
several latex agglutination assays for rapid id of S. aureus, that detect protein A and clumping factor. some detect capsular polysaccharide, which may improve ability to detect MRSA
Coagulase-negative Staphylococcus
only S. saprophytic is only relevant one in clinical medicine
usually don’t try to get species of these, if so, usually to reference lab
Staphylococcus Antimicrobial susceptibility
- more than 90% resistant to penicillin
- resistance to penicillin is due to an inducible plasmid-encoded B-lactamase, so sensitivity to penicillin should be confirmed after a period of induction with a B-lactam agent
resistance to penicillinase-resistant penicillin (methicillin, oxacillin, nafcillin) occurs in up to 80% of coagulase-nagative staph, and in more than 50% of isolates of hospital acquired S. aureus. mediated by the mecA gene, which encodes an altered penicillin-bidning protein (PBP-2a).
resistance is usually heterogeneous, meaning only rare cells are resistant
1 ug oxacillin dish for disk diffusion testing or a 30-ug cefoxitin disk may be better
> =22 mm is susceptible for coagulase positive
> =25 mm for coagulase negative
there are several assays for rapid detection of oxacillin resistance, including nucleic acid amplification, nucleic acid probe assays for mecA, and latex agglutination assays for PBP-2a.
for MRSA detection on nasal swabs, chromogenic media requires overnight incubation, and PCR-based assay can detect within 90 minutes. these can also be used for positive blood cultures.
although oxacillin-resistant staphylococci may appear to be susceptible to cephalosporins, they should be considered to be resistant to all B-lactam agents, including carbapenems.
vancomycin resistance is only rarely seen, but a serious issue and should be screened for
screen-positive isolates of vancomycin resistant strains should be sent to reference labs for confirmation and confirmed cases reported to state health department and the CDC.
newer antimicrobials have good activity against vancomycin resistant staphylococci. quinupristin/dalfopristin, daptomycin, linezolid, and most recently televancin.
