A. Typical Organisms

Staphylococci are spherical cells about 1 µm in diameter arranged in irregular clusters (Figure 1). 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. Under the influence of drugs such as penicillin, staphylococci are lysed.

Fig1. Gram-stain of Staphylococcus aureus showing Gram-positive cocci in pairs, tetrads, and clusters. Original magnification ×1000. (Courtesy of L. Ching.)

Micrococcus species often resemble staphylococci. They are found free living in the environment and form regular packets of four (tetrads) or eight cocci. Their colonies can be yellow, red, or orange. Micrococci are rarely associated with disease.

B. 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 (Figure 2). S. aureus usually forms gray to deep golden yellow colonies. S. epidermidis colonies usually are gray to white on primary isolation; many colonies develop pigment only upon prolonged incubation. No pigment is produced anaerobically or in broth. Various degrees of hemolysis are produced by S. aureus and occasionally by other species. Peptostreptococcus and Peptoniphilus species, which are anaerobic cocci, often resemble staphylococci in morphology. The genus Staphylococcus contains two species, Staphylococcus saccharolyticus and S. aureus subsp. anaerobius , which initially grow only under anaerobic conditions but become more aerotolerant on subcultures. This may be seen on rare occasions with some strains of S. epidermidis as well.

Fig2. Colonies of Staphylococcus aureus on a blood agar plate after 24 hours incubation. The yellow-gray colonies are 3–4 mm in diameter on the 10-cm plate. The colonies are surrounded by clear zones of hemolysis about 1 cm in diameter. (Courtesy of H. Reyes.)

C. Growth Characteristics

The staphylococci produce catalase, which differentiates them from the streptococci. Staphylococci slowly ferment many carbohydrates, producing lactic acid but not gas. Proteolytic activity varies greatly from one strain to another. Pathogenic staphylococci produce many extracellular sub stances, which are discussed below.

Staphylococci are relatively resistant to drying, heat (they withstand 50°C for 30 minutes), and 10% sodium chloride but are readily inhibited by certain chemicals (eg, 3% hexachlorophene).

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, piperacillin, and similar drugs). The plasmids are transmitted by transduction and perhaps also by conjugation.

2. Resistance to nafcillin (and to methicillin and oxacillin) is independent of β-lactamase production. Resistance to nafcillin is encoded and regulated by a sequence of genes found in a region of the chromosome called the staphylococcal cassette chromosome mec (SCCmec). Specifically, the mecA and mecC genes on this locus encode a low affinity penicillin-binding protein (PBP2a) that is responsible for the resistance. There are 12 different SCCmec types. Types I, II, III, VI, and VIII are associated with hospital-acquired methicillin-resistant S. aureus (HA MRSA) infections and may contain genes that encode resistance to other antimicrobials as well. SCCmec type IV has principally been found in CA-MRSA strains that tend to be less resistant, more transmissible, and responsible for outbreaks in the United States and some countries in Europe. Types IX and X are associated with animals (live stock-associated MRSA [LA-MRSA]) of which type IX contains mecC. The other types have been limited to various geographic locations around the world.

 3. In the United States, S. aureus and S. lugdunensis are considered to be susceptible to vancomycin if the minimum inhibitory concentration (MIC) is 2 µg/mL or less; of intermediate susceptibility if the MIC is 4–8 µg/mL; and resistant if the MIC is 16 µg/mL or greater. Strains of S. aureus with intermediate susceptibility to vancomycin have been isolated in Japan, the United States, and several other countries. These are often known as vancomycin intermediate S. aureus (VISA). They generally have been isolated from patients with complex infections who have received prolonged vancomycin therapy. Often, there has been vancomycin treatment failure. The mechanism of resistance is associated with increased cell wall synthesis and alterations in the cell wall and is not caused by the van genes found in enterococci. S. aureus strains of intermediate susceptibility to vancomycin usually are nafcillin resistant but generally are susceptible to oxazolidinones and to quinupristin-dalfopristin.

4. Since 2002, several isolates of vancomycin-resistant S. aureus (VRSA) strains (MICs ≥ 16 µg/mL) have been isolated from patients in the United States. The isolates contained the vancomycin resistance gene vanA likely derived from enterococci  and the nafcillin resistance gene mecA. Both of the initial VRSA strains were susceptible to other antibiotics. Vancomycin resistance in S. aureus is of major concern worldwide.

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 by it; that is, there is great difference between minimal inhibitory and minimal lethal concentrations of an antimicrobial drug. Patients with endocarditis caused by a tolerant S. aureus may have a prolonged clinical course compared with patients who  have endocarditis caused by a fully susceptible S. aureus. Tolerance can at times be attributed to lack of activation of autolytic enzymes in the cell wall.

D. Variation

 A culture of staphylococci contains some bacteria that differ from the bulk of the population in expression of colony characteristics (colony size, pigment, hemolysis), in enzyme elaboration, in drug resistance, and in pathogenicity. In vitro, the expression of such characteristics is influenced by growth conditions. 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. Some isolates may develop alterations in phenotypes such as smaller size (pin point colonies) and loss of hemolysis. These are referred to as small colony variants (SCVs) and the variations in phenotypic characteristics enable better survival under intracellular conditions, facilitating persistence and leading to chronic infections.

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