Staphylococcus aureus is the most virulent pathogenic species in the group
It is implicated in a variety of infections including skin, respiratory tract, post-op infections and other systemic infections such as TSS. S. aureus also causes food poisoning.
All Staph group species, including S. aureus, can lead a commensal existence (as an opportunistic pathogen) in the skin and mucous membranes of humans and many other animals, including domestic pets and farm animals.
S. intermedius is most often an inhabitant of dogs and S. hyicus is an inhabitant of swine
S. intermedius infections in humans are usually associated with dog bites (and are probably identified incorrectly as S. aureus since the human clinical laboratories rely primarily on the coagulase test)
Micrococcus species and R. mucilaginous are rarely pathogenic but must be differentiated from phenotypically similar Staphylococcus species
Staphylococcus species can be differentiated from Micrococcus species based upon oxygen requirements: Staph is facultative and Micrococcus species are obligate aerobes
Rothia can be differentiated from Staphylococcus and Micrococcus by its lack of growth on a high-salt medium, its negative catalase reaction, and its tendency to adhere firmly to an agar surface
Presumptive Genus Identification table not on test 6.5% Furazoli- Modified 0.04 U Salt done oxidase Bacitracin Micrococcus + R + S Staphylococcus + S - R R. mucilaginosa - NT NT NT Not included: oxygen requirements www.freelivedoctor.com
Coagulase (staphylocoagulase) is a fibrinogen activating enzyme produced by some staph species - it has thrombin-like activity. In situ, coagulase combines with “coagulase reacting factor” (CRF) to catalyze the formation of fibrin clots around cells as a barrier to host immune components – it is a virulence factor.
Clinically significant staphylococci are usually divided into two groups: those that produce coagulase and those that do not
Coagulase positive species include S. aureus, S. intermedius and S. hyicus
S. intermedius and S. hyicus mostly inhabit animals and are only rarely found as a cause of human infections
Coagulase Staphylococcus aureus coagulation of plasma* “ Free” staphylocoagulase Fibrinogen + CRF Fibrin Normal coagulation of plasma Thrombin Fibrinogen Fibrin (soluble ) (insoluble) *EDTA Rabbit plasma is preferred for the “free” or tube coagulase test because it contains a large amount of CRF www.freelivedoctor.com
Clumping of the suspension within 10 seconds indicates a positive test
Human plasma is preferable to rabbit plasma for the slide coagulase test because it yields more consistent results.
Note Your textbook indicates that rabbit plasma is used for both coagulase test; this is contrary to the Staphylococcus chapter in the “bible” (The Manual of Clinical Microbiology published by The American Society for Microbiology)
Protein A is unique to, and is an integral part of the cell wall surface of S. aureus . Protein A has an anti-phagocytic property (a virulence factor).
Protein A also has the unusual ability to bind specifically to Fc fragments of IgG (a sort of antigen-antibody reaction) from several species of animals, including Homo sapiens. This makes it well adapted for another test …
A diagnostic tool utilizes IgG adsorbed to some visible inert particle (such as latex beads) forming the basis of another clumping test for identifying S. aureus called the protein A clumping test. If you remember from Micro, this is an example of what we called an agglutination-type test.
IgG coated particles are mixed with cells taken from an agar culture. The complex forms within 10 seconds in the presence of S. aureus appearing as large granular-firm (not stringy) clumps
This test is claimed to be about 99% specific and sensitive for S. aureus
Protein A Latex Test S S S S S S L L L L L IgG S= S.aureus with Protein A L=Latex particle Protein A S Fc S www.freelivedoctor.com
Some reports in the literature indicate that an occasional methicillin resistant S. aureus (MRSA) will give a negative protein A/clumping factor test
Authors of these reports recommend that isolates that resemble colonies of S. aureus giving a negative protein A/clumping factor test and are also resistant to methicillin be tested by the tube coagulase method
The Protein A test should only be used on isolates that presumptively ID (morphology, catalase, salt tollerance, mannitol fermentation, etc) as S. aureus since some organisms give a false positive reaction ( Enterococcus, Micrococcus and rare strains of Staphylococcus saprophyticus ). These false positive reactions are slower to develop, with clumps that are smaller and have a “stringy” consistency.
A tube coagulase test should be ran on all presumptive S. aureus that give a negative Protein A test.
Another characteristic of most S. aureus cultures is the production of a heat stable enzyme that hydrolyzes RNA or DNA - a “nuclease”.
Nucleases are heat stable, able to withstand 15 minutes of boiling water. Testing to see if the boiled growth medium contains an active enzyme that hydrolyzes DNA or RNA is called a thermo nuclease test
Nucleases that may be produced by most Staphylococcus species other that S. aureus are not stable after boiling (if they do produce a nuclease it is not resistant to heat)
Clinical samples rarely contain pure cultures - should be assumed mixed. For this reason, culture on a selective medium for Gram positive bacteria such as Mannitol salt agar (MSA) or Colistin-Nalidixic Acid Agar (CNA).
MSA: notice growth and agar turns yellow around colonies.
Also conduct primary culture on sheep blood agar (SBA). See colonies that progress from small, covex and off-white to larger flatter opaque “porcelain” golden yellow colonies. Notice beta hemolysis.
Innoculate plate by rolling swab (if culture is on swab) on surface of agar in the first quadrant, then streak the remaining quadrants for isolation with loop. Incubate 35 o C for 24 hours.
Preliminary ID: catalase positive, gram-positive coccus in tetrads and clusters.
Additional: characteristic behavior on media and positive coagulase test. Can also use automated ID system and serological methods.
Many bacterial species and viruses alike cause some manner of upper respiratory tract (URT) infection.
S. aureus URT infection is fairly common with strep throat-like symptoms. Can co-reside with S. pyogenes or respiratory viruses such as influenza or RSV. Often causes secondary infections following respiratory viral infection.
Uncommon cause of accute sinusitis and otitis media
S. aureus is an uncommon cause of community acquired pneumonia (both primary and secondary) which is a lower respiratory tract (LRT) condition, although nosocomial cases are not uncommon. In fact, a CDC study in 1990 said S. aureus was the #1 cause of nosocomial pneumonia!
These cases have a high mortality rate due to the immune compromised status of the patient, and high degree of antibiotic resistance of strains in the hospital. Comments made above about S. aureus co-residing and secondary infections apply here as well.
Over 50% of community acquired “typical” bacterial pneumonias are caused Streptococcus pneumoniae - #1 cause
S. aureus is a common resident of the skin and exposed mucus membranes: respiratory, genitourinary & gastrointestinal.
S aureus is the most common cause of pathogenic integument infection in humans.
S. aureus is the #1 cause of post-operative infection, whether it be introduced during the course of the operation or afterward. These initial infections often become systemic and have high mortality rates
Less severe integumentary cases include styes, pimples, folliculitis, and other localized absecces.
Folliculitis (infected hair follicles) can become more deep seated causing a “furuncle” (a.k.a. boil). Multiple furuncles coalesce into a carbuncle. In severe, case S. aureus can spread hematogenously from here to any body site.
S. aureus & S. pyogenes cause impetigo, the most common skin infection in children – highly contagious
Also causes scalded-skin syndrome (Ritters syndrome) in infants via production of an exfoliating toxin – fairly rare, at least in US
*S. aureus is the classic cause of “toxic shock syndrome,” a highly acute and highly toxigenic condition – “super-infection” & “super-antigens” result in organ destruction, shock, hypotension and death
S. aureus possesses many properties that contribute to its ability to cause disease
Not all strains of S. aureus possess all of these virulence factors but most possess several:
Capsule: Like many strains of S. epidermidis , S. aureus produces a slime layer that adheres firmly to “plastic” prosthetic devices like catheters, shunts, and plastics bags used for continuous ambulatory peritoneal dialysis (CAPD). Capsules are also anti-phagocytic
Penicillin resistance (possession of penicillnase) is coded on a plasmid – the enzyme is also known as beta-lactamase: inactivates the beta-lactam ring of penicillins and other beta-lactam antibiotics such as the cephalosporins
Semi-synthetic drugs (modified penicillins) such as methicillin and oxacillin were developed for treating beta lactamase positive S. aureus infections
Some strains are now resistant to these drugs – MRSA, etc. A recent survey indicated that as many as 34% of S. aureus isolates were MSRA.
MRSA is not only resistant to methicillin and most other penicillins and cephalosporins, but they are often resistant to almost all other antibiotics except vancomycin
Although vancomycin is the drug of choice for treating MRSA infections, there are now vancomycin resistant (VRSA) S. aureus strains. The first VRSA strain was identified in Japan in 1997, and 8 cases were confirmed in the US in 2002.
A high percentage of CoNS are also resistant to methicillin. Even though these isolates may be responsible for a variety of infections, methicillin resistant varieties are no more likely to be associated with nosocomial infections than are methicillin sensitive isolates.
Hospital and nursing home epidemiological surveillance programs are routinely conducted for MRSA but NOT for methicillin resistant coagulase negative staphylococci
Rothia mucilaginosus is most abundant in the oral cavity
Microscopically Rothia species form short chains and small clusters. Here, the longer axes are perpendicular rather than parallel
The texture of R. mucilagenosus colonies is unique:
When pressure is applied using a loop, needle, or wooden applicator stick, R. mucilagenosus adheres tenaciously to the agar surface
When further pressure is applied the upper portion of the colony “peels off ” leaving the bottom portion sticking to the agar surface
The most reliable test for differentiating R. mucilaginosus from staph and micrococci is strict salt sensitivity on salt agar (e.g.5-7.5% NaCl) such as mannitol salt agar. Micrococcus doesn’t grow well, but it grows a little bit.
Presumptive Genus Identification table not on test 6.5% Furazoli- Modified 0.04 U Salt done oxidase Bacitracin Micrococcus + R + S Staphylococcus + S - R R. mucilaginosa - NT NT NT www.freelivedoctor.com