All About Staphylococci

The staphylococci are gram-positive spherical cells, usually arranged in grapelike irregular clusters. They grow readily on many types of media and are active metabolically, fermenting carbohydrates and producing pigments that vary from white to deep yellow.

The genus Staphylococcus has at least 40 species. The four most frequently encountered species of clinical importance are Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus lugdunensis  and Staphylococcus saprophyticus.

S aureus is coagulase positive , which diff erentiates it from the other species. S aureus is a major pathogen for humans. Almost every person will have some type of S aureus infection during a lifetime, ranging in severity from food poisoning or minor skin infections to severe life-threatening infections.

Approximately 75% of these infections caused by coagulase-negative staphylococci are caused by S epidermidis; infections caused by Staphylococcus lugdunensis, Staphylococcus warneri, Staphylococcus hominis.

Morphology and identification

Typical Organisms

- Staphylococci are spherical cells about 1 μm in diameter arranged in irregular clusters

- Single cocci, pairs, tetrads, and chains are also seen in liquid cultures.

- Young cocci stain strongly gram positive; on aging, many cells become gram negative.

- Staphylococci are nonmotile and do not form spores.

Culture                             

- Staphylococci grow readily on most bacteriologic media under aerobic or microaerophilic conditions. 

- They grow most rapidly at 37°C but form pigment best at room temperature (20–25°C).

- Colonies on solid media are round, smooth, raised, and glistening.

- S aureus usually forms gray to deep golden yellow colonies.

- S epidermidis colonies usually are gray to white on primary isolation.

 Growth Characteristics

- The staphylococci produce catalase, which diff erentiates them from the streptococci.

- Staphylococci slowly ferment many carbohydrates, producing lactic acid but not gas.

- Staphylococci are relatively resistant to drying, heat and 9% sodium chloride but are readily inhibited by certain chemicals

- Staphylococci are variably susceptible to many antimicrobial drugs. Resistance is caused by several mechanisms: 

1. β-Lactamase production is common, is under plasmid control, and makes the organisms resistant to many penicillins (penicillin G, ampicillin, ticarcillin, piperacillin) 
2. Resistance to nafcillin 
3. In the United States, S aureus and S lugdunensis are con Sidered to be susceptible to vancomycin if the minimum inhibitory concentration 
4. Since 2002, several isolates of vancomycin-resistant S aureus (VRSA) strains were isolated from patients in the United States. 
5. Plasmid-mediated resistance to tetracyclines, erythromycins, aminoglycosides, and other drugs is frequent in staphylococci. 
6. “Tolerance” implies that staphylococci are inhibited by a drug but not killed

Variation

When nafcillin-resistant S aureus is incubated at 37°C on blood agar, one in 107 organisms expresses nafcillin resistance; when it is incubated at 30°C on agar containing 2–5% sodium chloride, one in 103 organisms expresses nafcillin resistance.

Antigenic Structure

- Staphylococci contain antigenic polysaccharides and proteins as well as other substances important in cell wall structure.

- Peptidoglycan, a polysaccharide polymer containing linked subunits, provides the rigid exoskeleton of the cell wall.

- Teichoic acids, which are polymers of polyribitol– phosphate, are cross-linked to the peptidoglycan and can be antigenic.

- Protein A is a cell wall component of S aureus strains and is a bacterial surface protein that has been characterized among a group of adhesins called microbial surface components recognizing adhesive matrix molecules (MSCRAMMS).

Enzymes and Toxins

Catalase

-Staphylococci produce catalase, which converts hydrogen peroxide into water and oxygen.

-The catalase test differentiates the staphylococci, which are positive, from the streptococci, which are negative.

Coagulase and Clumping Factor

- S aureus produces coagulase, an enzyme-like protein that clots oxalated or citrated plasma. Coagulase binds to prothrombin. Coagulase may deposit fibrin.

- Clumping factor is another example of an MSCRAMM that is responsible for adherence of the organisms to fibrinogen and fibrin.

Hemolysins              

- The α-Hemolysin is a heterogeneous protein that acts on a broad spectrum of eukaryotic cell membranes.

- The β-toxin degrades sphingomyelin and therefore is toxic for many kinds of cells, including human red blood cells.

-The δ-toxin is heterogeneous and dissociates into subunits in nonionic

detergents.

- The γ hemolysin is a leukocidin that lyses white blood cells.

Panton-Valentine Leukocidin

- This toxin of S aureus has two components, and unlike the chromosomally encoded hemolysins above, PVL is encoded on a mobile phage.

- It can kill white blood cells of humans and rabbits.

Exfoliative Toxins

- These epidermolytic toxins of S aureus are two distinct proteins of the same molecular weight.

- Exfoliative toxin A is encoded by eta located on a phage and is heat stable (resists boiling for 20 minutes).

-Exfoliative toxin B is plasmid mediated and heat labile.

Toxic Shock Syndrome Toxin

- TSST-1 binds to major histocompatibility class (MHC) class II molecules, yielding T-cell stimulation, which promotes the protean manifestations of the toxic shock syndrome.

- The toxin is associated with fever, shock, and multisystem involvement, including a desquamative skin rash.

Enterotoxins

-Important causes of food poisoning, entero Toxins are produced when S aureus grows in carbohydrate and protein foods.

- Ingestion of 25 µg of enterotoxin B results in vomiting and diarrhea.

Pathogenesis

- Staphylococci, particularly S epidermidis, are members of the normal microbiota of the human skin and respiratory and gastrointestinal tracts.

- Nasal carriage of S aureus occurs in 20–50% of humans.

- Staphylococci are also found regularly on clothing, bed linens, and other fomites in human environments.

- The pathogenic capacity of a given strain of S aureus is the combined effect of extracellular factors and toxins together with the invasive properties of the strain.

- At one end of the disease spectrum is staphylococcal food poisoning, attributable solely to the ingestion of preformed enterotoxin; at the other end are staphylococcal bacteremia and disseminated abscesses in all organs.

- Pathogenic, invasive S aureus produces coagulase and tends to produce a yellow pigment and to be hemolytic.

- Nonpathogenic, noninvasive staphylococci such as S epidermidis are coagulase negative and tend to be nonhemolytic.

Clinical Findings

- A localized staphylococcal infection appears as a “pimple,” hair follicle infection, or abscess.

- S aureus infection can also result from direct contamination of a wound.

- If S aureus disseminates and bacteremia ensues, endocarditis, acute hematogenous osteomyelitis, meningitis, or pulmonary infection can result.

- Food poisoning caused by staphylococcal enterotoxin is characterized by a short incubation period (1–8 hours); violent nausea, vomiting, and diarrhea

- Toxic shock syndrome is manifested by an abrupt onset of high fever, vomiting, diarrhea, myalgias, a scarlatiniform rash, and hypotension with cardiac and renal failure in the most severe cases

Diagnostic Laboratory Tests

Specimens

- Surface swab pus or aspirate from an abscess, blood, tracheal aspirate, or spinal fluid for culture, depending on the localization of the process.

Smears

- Typical staphylococci appear as gram-positive cocci in clusters in Gram-stained smears of pus or sputum.

Culture

- Specimens planted on blood agar plates give rise to typical colonies in 18 hours at 37°C.

- Specimens contaminated with a mixed flora can be cultured on media containing 7.5%

NaCl; the salt inhibits most other normal microbiota but not S aureus.

Catalase Test

- This test is used to detect the presence of cytochrome oxidase enzymes.

- A drop of 3% hydrogen peroxide solution is placed on a slide, and a small amount of the bacterial growth is placed in the solution.

Coagulase Test

- Citrated rabbit (or human) plasma diluted 1:5 is mixed with an equal volume of broth culture or growth from colonies on agar and incubated at 37°C.

Treatment

- Bacteremia, endocarditis, pneumonia, and other severe infections caused by S aureus require prolonged intravenous therapy with a β-lactamase-resistant penicillin.

- Vancomycin is often reserved for use with nafcillin-resistant staphylococci.

- Penicillin G–resistant S aureus strains from clinical infections always produce penicillinase.

- They constitute more than 95% of S aureus isolates in communities in the United States.

- They are often susceptible to β-lactamase–resistant penicillins, cephalosporins, or vancomycin.