Staphylococcus And Others Flashcards
General Media of choice
-Blood agar: this is the #1 choice for staph. It shows hemolysis and pigment
-PEA or CNA: selects for gram positive. Used when a swarming proteus species is present
-Mannitol salt: contains a high salt concentration and shows mannitol formation w/phenol red as the pH indicator
-Staph Aureus is yellow (ferments mannitol)
-MRSA screen: selective for MRSA: contains ceofixitin, staph aureus will turn pink and all other staph will turn blue/green
Staphylococcus or Micrococcus?
-Staph: Gram positive cocci in clusters on a gram stain, microdase negative, bacitracin resistant, catalase positive, white or buff colony on blood agar with butyrous growth (butter), yellow pigment due to -lipochrome production
-Micrococcus: gram positive cocci in tetrads on a gram stain, microdase (modified oxidase) positive (blue), bacitracin susceptible, bright lemony yellow pigment, normal skin flora
Catalase test: Staph or Strep?
-staph is catalase positive
-Smear small bit of growth on glass slide, add hydrogen peroxide
-Positive= bubbles, negative = no bubbles
-False positives can occur due to red cells in the blood agar, using an iron loop to mix the organism into the h2O2, or having a mixed culture
-Pseudocatalase: produced by some organisms like Enterococcus and may appear very weakly positive, read quickly to avoid false positive
Is Coag Neg Staph a contaminant or pathogen?
-usually normal skin and mucosa flora
-For blood cultures: if
-1 set is positive= its probably a contaminant
-Multiple sets are positive: it’s probably a pathogen
Staph Aureus or Coag neg Staph?
-Tube coagulase (free coagulase): emulsify colony in 0.5mL rabbit plasma and incubate for 24 hrs in a non -CO2 incubator: Clot formation (solid) is positive = SA, no clot formation (liquid) is negative: CNS
-Clumping factor/staph latex (bound coagulase): faster, mix bacteria and rabbit plasma on a slide: positive is -clumping = SA, negative is no clumping = CNS
-The tube coagulase method is the reference test for this
-Hemolysis: Staph aureus is beta hemolytic, CNS is usually non-hemolytic (gamma) on blood agar
Staph Aureus toxins
-Protein A: cell wall protein that helps SA evade the immune system
-Enterotoxins: type of exotoxin that affects the GI tract and is what causes food poisoning due to SA
Common infections caused by Staph Aureus
-folliculitis, furuncles
-Serious: scalded skin syndrome (exfoliative toxin), toxic shock (TSST-1 toxin)
-Lung and UTIs
-food poisoning (symptoms within 2-6 hours of ingestion): caused by ingesting enterotoxin
-Deep site: bone, joint, heart valve, spleen and liver
-⅔ are HAI in surgical wound infections, catheters, grafts, prosthetics, shunts
-Community acquired MRSA: skin to skin contact (lives in nasal passages)
Beta-lactamase
-involves MRSA
-Enzyme that breaks the beta lactam ring found in penicillin, cephalosporins, and methicillin, and deactivates these antibiotics
-Detect the production of this enzyme via simple disk test
-For MRSA, vancomycin is the antibiotic treatment
PB2A production via MRSA
-penicillin binding protein 2a: a mutated cell wall protein encoded by the gene mecA that allows staph to be methicillin resistant (beta lactam agents can’t bind as well)
MALDI-Tof to differentiate between MRSA and SA
-Uses culture based assays to differentiate between the two and can also provide information regarding strain virulence
-can basically differentiate colonies on an agar plate essentially
CNS infections
-opportunistic infections: like in dwelling catheters, shunts, lines
-Staph lugdenesis: PYR positive (used for presumptive identification of group A beta hemolytic strep and enterococci), other CNS infections are PYR negative
-Staph epidermidis: grey colonies on BA, cause of skin and peritoneal dialysis infections
-Staph saprophyticus: cause of UTIs in young, sexually active females ages 10-45, bright pearly white on BA, novobiocin disk resistant
Novobiocin test for Staph Saprophyticus
-Identified as this organism if it proves to be resistant to novobiocin
-A novobiocin disk is placed in the middle of the agar and if the resulting zone of inhibition is less than 16mm then it is positive for saprophyticus
Genus Stomatococcus
-are encapsulated which makes them sticky on agar, coag negative, normal oral flora,
-Disease in wound infections of immunocompromised people and drug abusers
-Gram positive cocci
Vancomycin
-a good antibiotic for MRSA since it is resistant to penicillin, methicillin, oxacillin, naficillin, and cefotoxitin, which are used to treat MSSA
