Staphylococcus aureus Flashcards
what are the bacterial cell characteristics of Sa?
- gram positive cocci
- either exist as single cells or grape-like clusters
- nonmotile, noncapsular
- non-spore forming, sensitive to heat treatment
- facultative anaerobes but grow rapidly under aerobic conditions
- mesophiles and can grow between 7-48C, and rapidly between 20-37C
- can grow at relatively low Aw (0.86), low pH (4.8) and under high salt and sugar conditions
- can grow under several adverse conditions, but are poor competitors against other organisms, therefore they do very well when adverse environments give them a competitive advantage
foodborne illness is usually an …
intoxication
what do strains of Sa that cause foodborne illness carry?
an enterotoxin
how is Sa characterized based off of sequence type or allelic profile?
multilocus sequencing typing, based on the sequences of 7 housekeeping genes found in all strains of Sa
how are groups of sequence types defined if groups share at least 5 of 7 alleles?
defined as a clonal complex (CC)
how is Sa distributed among the hosts they are able to infect?
distributed ubiquitously among mammalian, reptile, and avian hosts
certain lineages are specific to certain hosts
what clonal complexes are bovine infections commonly caused by?
CC20, CC188, CC8, CC97, and CC151
what are the predominant CC’s in human colonization and infection?
CC1, CC5, CC8, CC30, and CC45
most strains capable of producing the enterotoxin are from the human CCs
what is the Sa accessory genome made up of?
several mobile genetic elements which can be transferred horizontally (20% of the genome)
how is horizontal gene transfer inhibited in Sa lineages?
due to a very advanced restriction modification system in Sa, preventing HGT between different clonal complexes
how many prophage genomes do Sa genomes carry?
1-4, phage genomes have enterotoxins
what genetic elements are many of the enterotoxins located on?
mobile genetic elements
definition of enterotoxin:
a toxin that acts on the GI tract
definition of endotoxin:
a toxin that is structurally associated with the bacterial cell (LPS)
definition of exotoxin:
a toxin that is usually secreted into the extracellular fluid but can be intracellular or injected directly into eukaryotic cells
definition of exfoliative toxins:
dermolytic exotoxins produced exclusively by Sa that cause the symptoms of impetigo or scalded skin syndrome
all Sa enterotoxins are also ….
exotoxins
how are all Sa enterotoxins similar?
all have a high degree of homology, and all cause gastroenteritis
what growth phase are enterotoxins primarily made during?
exponential growth phase
how can enterotoxin production be inhibited?
by the presence of glucose
what other function can enterotoxins of Sa be involved in?
quorum sensing
how does Sa compare to the rest of the non-spore forming pathogens?
it is one of the most resistant to environmental stress. it is also very osmotolerant which is problematic because there are few bacteria that can compete with it in these environments
staphylococcal enterotoxins are primarily produced during …
the late exponential or early stationary phase
what system are some SE’s regulated by?
the agr system that is a density-sensing sytem in Sa
what are the proteins that form a quorum-sensing system
AgrA - AgrD
what is the auto-inducing peptide?
peptide produced to be recognized by quorum-sensing system, specifically AgrC to initiate signal transduction pathways
what happens once AgrC recognizes AIP?
AgrC then phosphorylates AgrA and activate the expression of RNAIII
how are SEB and SEC regulated?
regulation via agr can be translational or transcriptional - SEB and SEC are regulated at the transcriptional level.
Repressor of Toxin (ROT) negatively regulates transcription of SEB by binding to the promoter region (RNAIII binds to ROT and causes degradation thus up-regulating SEB)
how does SarA up-regulate SEB?
binds to several promoters including agr and upregulates RNAIII
what is SEA linked to?
the life cycle of temperate phage:
- has two promoters, one immediately upstream of the sea gene and the other at the phage-related latent promoter
- during stress induced phage induction, both promoters are activated resulting in lots of production of SEA
what is the main environmental source of Sa?
people
- we have it on our skin, the interior of the nose is the main site, it spreads by direct contact, skin fragments, and respiratory droplets
- also found in erupted acne
- able to invade our cells and live intracellularly
what environments are Sa able to survive in?
a variety of extreme environments and is isolated from air, dust, sewage, and water. can also acquire resistance to heavy metals and antibiotics
how does Sa affect animals?
- often a cause of bovine mastitis, and a variety of infections in agricultural animals
what is the impact on farms when a cow has bovine mastitis?
when milked, the bacteria can contaminate milk and dairy products. this cow can also contaminate other batches of milk. if the toxins resulting from this contamination can grow, the toxins may survive pasteurization
in cases of food contamination with Sa, where is it mostly traced back to?
humans, during food preparation
what are the contributing factors of Sa food-contamination?
- inadequate refrigeration
- preparing foods too far in advance
- poor personal hygiene
- inadequate cooking
- prolonged use of warming plates (i.e. buffet)
Characteristics of Sa disease:
- very quick onset (30 min), mean incubation is 4.4 hours, 10 hrs is maximum time to onset
- self-limiting and recovery in 24-48 hours
- most cases not reported
- outbreaks are seasonal, many of the cases/outreaks occur during the summer
- there is never any fever
- almost everyone vomits, other symptoms can have nausea, diarrhea, and abdominal pain
- death is very rare
- purified toxin is considered a bioterrorism agent
what is methicillin-resistant Sa (MRSA)? where is it found?
- strains that carry the mecA gene
- causative agent of soft tissue and skin infections, along with necrotizing fasciitis
- found in cows, pigs, and 0.4-12% of retail meat (carrying the enterotoxin gene)
prevention of Sa?
- hand washing, glove wearing, and avoidance of hiring food prep workers with active skin infections
- adequate refrigeration, or continued heating of prepared food. re-heating is not helpful
- preservatives could be used to kill or arrest growth of the organism
