Organism specific features Flashcards
What are the morphologic and culture features of Staphylococcus aureus?
Gram-positive cocci
catalase positive
0.5-1.5 µm in diameter, nonmotile, non-spore-forming, facultative anaerobes (with the exception of S. aureus anaerobius)
usually form in clusters.
Many strains produce staphylococcal TOXINS (enterotoxins, the superantigen toxic shock syndrome toxin (TSST-1), and exfoliative toxins)
*unique aspects = coagulase, protein A and species specific teichoic acid
Where is Staph aureus found normally?
Staphylococcus aureus are part of human flora, and are primarily found in the nose and skin
How does Staphylococcus aureus cause human disease?
Staphylococcus aureus is an opportunistic pathogen that can cause a variety of self-limiting to life-threatening diseases in humans.
- leading cause of food poisoning, resulting from the consumption of food contaminated with enterotoxins
- Animal bites can result in local infections, cellulitis, erythema, tenderness, mild fever, adenopathy, and lymphangitis (rarely)
- Impetigo
- hydradenitis suppurativa
- mastitis
- Scalded skin syndrome is caused by exfoliative toxins secreted on the epidermis and mostly affects neonates and young children
- Other skin conditions caused by Staphylococcal exfoliative toxins include blisters, skin loss, pimples, furuncles, impetigo, folliculitis, abscesses, poor temperature control, fluid loss, and secondary infection
- S. aureus can also cause necrotizing fasciitis in immunocompromised individuals, although this is very rare
- pneumonia (post-viral or post obstruction), empyema, pulm abscess
- UTI
What are some epidemiological characteristics of Staph aureus?
Worldwide distribution.
Staphylococcus aureus is one of the most common causes of skin, soft-tissue, and nosocomial infection
Rates of infection in community settings are increasing
Residents of nursing homes are also at an increased risk of acquiring MRSA
Around 20% of individuals are persistent carriers of Staphylococcus aureus, about 60% are intermittent carriers, and approximately 20% rarely carry it
Children are more likely to be persistent carriers of the bacteria.
Young women are at a higher risk for toxic shock syndrome
What is the infectious dose of staph aureus?
For the organism itself to cause illness (as opposed to toxin burden) –> 100,000 organisms
How is disease-causing staph aureus transmitted?
- somewhat trick question as it’s a common colonizer of the human…so usually mucosal breakdown or puncture
- TOXIN mediated disease = Ingestion of food containing enterotoxins
- Vertical transmission during vaginal delivery is uncommon
- Person-to-person transmission occurs through contact with a purulent lesion or with a carrier
- Unsanitary conditions and crowded community settings increase exposure to S. aureus.
- Infection may be spread from person-to-person through health care workers or patients
- Nasal colonization can lead to auto-infection
- biofilm forming organism so it can be on prosthesis that was implanted or a knife or something
To which drugs is staph aureus normally susceptible?
Antibiotics such as cloxacillin and cephalexin are commonly used to treat staph infections.
Vancomycin which is administered intravenously is used to treat MRSA
*the bulky R group penicillins (which are penicillinase-resistant) can treat staph (methicillin, oxacillin, nafcillin, dicloxacillin)
*resistance to these 4 means the strain is resistant to all beta-lactam antibiotics
What different toxins are produced by staph aureus that increase virulence?
(main idea) Cytotoxins are integral to colonies of staph aureus causing clinical disease –> they set up infection by secreting death and destruction all around them
*alpha toxin
*beta toxin/sphingomyelinase C
*gamma toxin
*P-V leukocidin
These toxins kill leukocytes, erythrocytes, fibroblasts, platelets, macrophages
(kehl level question) What does catalyse do?
catalase is an enzyme that converts hydrogen peroxide into water and oxygen gas. Drop peroxide solution on a colony and bubbles form if catalase is present
What are some other species of staphylococcus that are important to know about for contaminant vs pathogen discussions?
Staph capitis (on skin of head and face) Staph haemolyticus (apocrine gland colonizer) Staph hominis (apocrine gland colonizer) ***Staph lugdunensis (often a pathogen) Staph epidermidis Staph saprophyticus
Why do staph species tend to cluser?
Production of coagulase, which will convert fibrinogen to fibrin and form a tiny clot around themselves in serum. Some species don’t have coagulase and are collectively “coag-negative staph”
What diseases are most associated with staph aureus?
Toxin mediated diseases (food poisoning, scalded skin syndrome, toxic shock syndrome) Cutaneus diseases (carbuncles, folliculitis, furuncles, impetigo, wound infections *other (BLOOD STREAM INFECTIONS, endocarditis, pneumonia, empyema, osteomyelitis, septic arthritis)
What diseases are most associated with staph epidermidis?
Bacteremia (be careful --> often contaminant) endocarditis surgical wounds UTI catheter infections shunt infections prosthetic infections peritoneal dialysates
What dieseases are most associated with staph saprophyticus?
UTI (classically young female)
opportunistic infections
What diseases are most associated with staph lugdunensis?
(should remember this species as the most important coag negative staph. Recognize it as a pathogen) Endocarditis arthritis bacteremia opportunistic infections urinary tract infections
What diseases are most associated with staph haemolyticus?
*not often considered a pathogen Bacteremia endocarditis bone and joint infections UTI wound infections opportunistic infections
Can one get colonized by environmental staph aureus?
while they live predominantly on the skin of animals, they are rather tough mothers and can live on surfaces for a longer time than you think.
Is there any selective media you could use to recover staph aureus from contaminated or polymicrobial specimens?
YES, mannitol-salt agar can be used as staph aureus can survive the salt when other bugs can’t
