Staphylococci & Related gram + Flashcards
Describe the epidemiology of staphylococcal infection.
Found in external environment Found on skin and mucous membranes Anterior nares - 20-40% of adults Intertriginous skin folds Perineum Axillae Vagina Significant opportunistic pathogen under appropriate conditions
List virulence factors associated with S. aureus
Capsules Protein A Panton-Valentine Leukocidin (PVL) Coagulase Hemolysins (lysins, toxins) enzymes
Identify species of staphylococci that are coagulase and slide latex positive
S. aureus
List the clinically significant species of coagulase-negative staphylococci and describe the diseases they are associated with
Staphylococcus epidermidis (infections of inter dwelling devices) Staphylococcus saprophyticus ( acute UTIs) Staphylococcus lugdunensis (same as S. aureaus)
List six settings that have been associated with community-acquired MRSA outbreaks
Sports participants: football, wrestlers, fencers - MPSM **
Correctional facilities: prisons, jails
Military recruits
Daycare and other institutional centers
Newborn nurseries and other healthcare settings
Men who have sex with men - MSM **
List five severe disease syndromes in which MRSA should be considered in the differential diagnosis
- skin and soft tissue infections (SSTI’s)
- Sepsis syndrome
- Osteomyelitis *
- Necrotizing pneumonia
- Septic arthritis
- Necrotizing fasciitis *
Describe effective infection prevention measures to control MRSA infections in the hospital
** MRSA testing of all patients entering ICUs and contact precautions for all patients testing positive, produced: A 75% decrease in MRSA bacteremia in ICUs . And a 67% drop hospital wide **
Careful, compulsive hand hygiene for all patient interactions (behavioral change)
Standard and transmission based Contact/Droplet precautions:
Gowns Gloves Masks
Effective cleaning of the patient care environment
Clean shared/dedicated equipment
Stethoscopes BP cuffs
Thermometer TV Remotes
Appropriate use of antibiotics
what diseases are Micrococci associated with
none- not considered clinically significant when isolated from human specimens
Staphylococci Key Characteristics
Gram-positive cocci arranged single cells, pairs, tetrads and short chains, but mostly grape-like clusters
Non-motile
Non-spore-forming
***Catalase positive
Facultative anaerobes, except S. aureus subsp. anaerobius and S. saccharolyticus (these two also catalase negative)
Staphyloccoci are aerobic or anaerobic
Facultative anaerobes,
except S. aureus subsp. anaerobius and S. saccharolyticus (these two also catalase negative)
Staphylococci aureus
gram + with grape like clusters
blood agar plate - grow fairly large in 24 hrs, creme color, and a clearing zone around the colonies
found on skin and mucous membranes
Significant opportunistic pathogen under appropriate conditions
Staphylococcus aureus infections
Pyoderma Furuncle (or boil) carbuncle Scalded skin syndrome Toxic-shock syndrome Food poisoning Disseminated Infections
function of staph aureus’ capsule
prevent ingestion of organism by PMNs
Protein A function in staph aureus
binds Fc region of IgG, interfering with opsonization and ingestion of organism by PMNs
Panton-Valentine Leukocidin (PVL)
an enzyme that alters cation permeability of rabbit and human leukocytes resulting in white cell destruction
Coagulase
binds to prothrombin catalyzing conversion of fibrinogen to fibrin, which in turn acts to coat bacterial cells with fibrin, rendering them more resistant to opsonization and phagocytosis
α-hemolysin
Lyses RBCs of several animals
Dermonecrotic on subcutaneous injection
Leukocyte toxicity
β-hemolysin
Sphingomyelinase, varying lysis of RBCs from different animals due to differences in membrane sphingomyelin content
Produces “hot-cold” lysis (hemolysis enhanced at low temperature after 35 C incubation)
δ -hemolysin
Produced by 97% of S. aureus and 50-70% of coagulase negative Staph
Acts as surfactant that disrupts the cell membrane, interacts with membrane to form channels that increase in size over time resulting in leakage of cellular contents
Some coagulase-negative staphylococci produce enough delta toxin to cause NEC in neonates
γ-hemolysin
found in some S. aureus strains, also causes lysis of variety of cells
Exfoliatins or epidermolytic toxins
responsible for “staphylococcal scalded skin syndrome,” dissolves the mucopolysaccharide matrix of epidermis, causing separation of skin layers; rare in adults
Enterotoxins
heat-stable molecules responsible for clinical features of staphylococcal food poisoning, probably most common cause of food poisoning in U.S. Toxin produced in contaminated food by toxigenic strains, vomiting with or without diarrhea (2-8 hrs), quick recovery (24-48 hrs)
Fibrinolysins
break down fibrin clots and facilitate spread of infection to contiguous tissues
Hyaluronidase
hydrolyzes intercellular matrix of acid mucopolysaccharides in tissue acting to spread organisms to adjacent tissue
Phospholipase C
described in patients with ARDS and DIC. Tissues affected by this enzyme become more susceptible to damage and destruction by bioactive complement components and products during complement activation.
ARDS
acute respiratory distress syndrom
DIC
disseminating intracellular coagulation
pyrogenic toxin superantigens
Toxic shock syndrome toxin-1 (TSST-1) of S. aureus
Streptococcal pyrogenic exotoxins (SPE)
Streptococcal superantigens
characteristics of super antigens
Pyrogenicity
Superantigenicity
Enhance lethal effects of minute amounts of endotoxin
*All induce polyclonal T-cell proliferation
Latex Agglutination
Alternate coagulase test
latex beads coated with plasma. Fibrinogen bound to latex detects clumping factor. In addition, Ig molecules also on beads detect Protein A (staphylococcal cell-wall protein that binds IgG by the Fc region)
takes less than a minute, you look for the presents of beads = + (S. aureus)
Tube coagulase
free coagulase. Reacts with substance in plasma called coagulase-reacting factor that converts fibrinogen to fibrin
Staphylococcus epidermidis
- Most frequently isolated clinically significant coagulase-negative staphylococci
- normally a very common skin flora that is non-virulent
- Associated with infections of indwelling devices
- Virulence related to production of extracellular slime that promotes adherence of organism to surfaces of foreign bodies forming biofilm
- Biofilm also protects organisms from antimicrobial agents, therefore removal of foreign bodies often necessary for resolution of infection
Staphylococcus saprophyticus
- Cause of acute urinary tract infection in young women
- 2nd most common cause of uncomplicated cystitis (after E. coli) among women of college and child-bearing age
- Identification based on negative coagulase and resistance to novobiocin**
Staphylococcus lugdunensis
causes the same diseases as S. aureus
the only species that is both PYR and Ornithine Positive **
An organism identified as gram-positive cocci in clusters is isolated from the urine of a 21 year female with symptoms of acute cystitis. The laboratory reports that the organism is coagulase-negative, furozolidone susceptible and novobiocin resistant. What is the most likely identification of this bacterium?
Staphylococcus saprophyticus
who gets MRSA most commonly
patients in hospitals and healthcare facilities (such as nursing homes and dialysis centers) who have weakened immune systems
In hospitals, the most important reservoirs of MRSA are colonized or infected patients (self inflicted)
mecA Gene
- makes S. aureus MRSA
- Encodes for altered “penicillin-binding protein 2a” (PBP2a)
- Has decreased binding affinity for ß-lactam antibiotics (doesn’t allow penicillin like drugs to bind) and allows peptidoglycan synthesis even in the presence of B-lactam antibiotics
- mecA is carried on a mobile genetic element called “staphylococcal cassette chromosome mec” (SCCmec)
gene Xpert MRSA assay
real time PCR used to detect MRSA - gives results in under an hour from nasal swabs (both nares)
can be affected by: Whole blood, Mucus, and Nasal Spray
What is the most important reservoir of methicillin resistant Staphylococcus aureus (MRSA) in hospitals?
Colonized or infected patients
