Biology of the Staphylococci

The staphylococci

The staphylococci are gram-positive round cells, generally organized in grapelike abnormal clusters. They develop readily on many varieties of media and are lively metabolically, fermenting carbohydrates and generating pigments that vary from white to deep yellow. Some are individuals of the ordinary microbiota of the skin and mucous membranes of human beings; others reason suppuration, abscess formation, a spread of pyogenic infections, or even fatal septicemia. Th e pathogenic staphylococci often hemolyze blood, coagulate plasma, and bring a spread of extracellular enzymes and toxins. Th e maximum common form of food poisoning is because of a heat stable staphylococcal enterotoxin. Staphylococci swiftly increase resistance to many antimicrobial agents, which therefore offers difficult therapeutic troubles. Th e genus Staphylococcus has at least 40 species. Th e four maximum frequently encountered species of clinical importance are Staphylococcus aureus, Staphylococcus epidermis, Staphylococcus lugdunensis , and Staphylococcus saprophyticus. S aureus is coagulase positive , which differentiates it from the other species. S aureus is the main pathogen for human beings. Almost anyone can have some form of S aureus infection at some stage in a lifetime, ranging in severity from food poisoning or minor pores and skin infections to intense life-threatening infections. Th e coagulase-positive staphylococci are ordinary human microbiota and occasionally motive infection, oft en related to implanted organs, such as joint prostheses, shunts, and intravascular catheters, particularly in very young, old, and immunocompromised patients. Approximately seventy-five percentage of those infections due to coagulase-negative staphylococci are due to S epidermidis; infections resulting from Staphylococcus lugdunensis, Staphylococcus warneri, Staphylococcus hominis, and different species are much less common. S saprophyticus is an especially commonplace reason of urinary tract infections in young girls, even though it hardly ever causes infections in hospitalized patients. Different species are critical in veterinary medicine.

Morphology and identification

1.Typical Organisms

Staphylococci are spherical cells approximately 1 m in diameter arranged in irregular clusters ( discern 13-1 ). Single cocci pairs, tetrads, and chains also are seen in liquid cultures. younger cocci stain strongly gram positive quality; on getting older, many cells end up gram negative. Staphylococci are nonmotile and do not shape spores. Beneath the influence of drugs consisting of penicillin, staphylococci are lysed. Micrococcus species often resemble staphylococci. Th ey are determined unfastened residing in the surroundings and from normal packets of 4 (tetrads) or 8 cocci. Th eir colonies can be yellow, crimson, or orange. Micrococci are hardly ever associated with disease.

2.Culture

Staphylococci grow effectively on maximum bacteriologic media below aerobic or microaerophilic conditions. Th ey grows most swiftly at 37ï‚°C but shape pigment excellent at room temperature (20–25ï‚°C). Colonies on strong media are spherical, clean, raised, and glistening . S aureus typically gives gray to deep golden yellow colonies. S epidermidis colonies commonly are gray to white on primary isolation; many colonies broaden pigment handiest upon extended incubation. No pigment is produced anaerobically or in broth. Various degrees of hemolysis are produced via S aureus and from time to time by using other species. Peptostreptococcus and Peptoniphilus species, which might be anaerobic cocci, often resemble staphylococci in morphology. Th e genus Staphylococcus contains species, S saccharolyticus and S aureus subsp. anaerobius, which initially grow best below anaerobic situations but grow to be greater aerotolerant on subcultures. This can be seen on rare occasions with a few strains of S epidermidis as properly.

3.Colony characteristics

Th e staphylococci produce catalase, which differentiates them from the streptococci. Staphylococci slowly ferment many carbohydrates, producing lactic acid but no longer gas. Proteolytic activity varies greatly from one strain to every other. Pathogenic staphylococci produce many extracellular materials, which are discussed below. Staphylococci are tremendously proof against drying, warmness (they withstand 50ï‚°C for 30 minutes), and nine% sodium chloride but are simply inhibited with the aid of certain chemical substances (eg, 3% hexachlorophene). Staphylococci are variably at risk of many antimicrobial pills. Resistance is as a result of several mechanisms:

1. -Lactamase production is common, is below plasmid manipulate, and makes the organisms resistant to many penicillins (penicillin G, ampicillin, ticarcillin, piperacillin and comparable capsules). The plasmids are transmitted through transduction and possibly also by using conjugation.
2. Resistance to nafcillin (and to methicillin and oxacillin) is independent of -lactamase manufacturing. Resistance to nafcillin is encoded and regulated through a sequence of genes found in a place of the chromosome referred to as the staphylococcal cassette chromosome mec (SCCmec). Particularly, the mecA gene in this locus encodes a low-affinity penicillin binding protein (PBP2a) that is answerable for the resistance. There is 12 one-of-a-kind SCCmec types. types I, II, and III are associated with medical institution-acquired infections and might include genes that encode resistance to different antimicrobials as properly. SCCmec type IV has principally been found in network-acquired methicillin-resistant S aureus (CA-MRSA) lines that tend to be much less resistant, more transmissible, and responsible for outbreaks over the past decade within the USA and a few nations in Europe. The alternative types had been limited to diverse geographic places round the sector.
3. In the u.s., S aureus and S lugdunensis are taken into consideration to be susceptible to vancomycin if the minimal inhibitory awareness (MIC) is 2 g/mL or less; of intermediate susceptibility if the MIC is 4–8 g/mL; and resistant if the MIC is sixteen g/mL or greater. Lines of S aureus with intermediate susceptibility to vancomycin had been isolated in Japan, the united states, and several other nations. those are often called vancomycin-intermediate S aureus (VISA). They typically had been isolated from patients with complex infections who have received extended vancomycin remedy. Frequently there was vancomycin remedy failure. The mechanism of resistance is related to expanded cell wall synthesis and alterations inside the cellular wall and isn't as a result of the van genes discovered in enterococci. S aureus strains of intermediate susceptibility to vancomycin usually are nafcillin resistant however normally are at risk of oxazolidinones and to quinupristin–dalfopristin.

4. For the reason that 2002, numerous isolates of vancomycin-resistant S aureus (VRSA) strains have been remoted from patients in the USA. The isolates contained the vancomycin resistance gene vanA from enterococci and the nafcillin resistance gene mecA. each of the preliminary VRSA lines had been vulnerable to different antibiotics. Vancomycin resistance in S aureus is of most important international concern.
5. Plasmid-mediated resistance to tetracyclines, erythromycins, aminoglycosides, and different capsules is common in staphylococci.
6. “Tolerance” means that staphylococci are inhibited by way of a drug however no longer killed by using it that is, there may be the primary difference between minimal inhibitory and minimum deadly concentrations of an antimicrobial drug. Patients with endocarditis as a result of a tolerant S aureus might also have a prolonged scientific course in comparison with patients who have endocarditis caused by a completely inclined S aureus. Tolerance can at times be attributed to a loss of activation of autolytic enzymes inside the cell wall.

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4.Variant

A subculture of staphylococci includes a few microorganism that differs from the bulk of the population in an expression of colony characteristics (colony size, pigment, hemolysis), in enzyme elaboration, in drug resistance, and in pathogenicity. In vitro, the expression of such traits is stimulated with the aid of increase situations: while nafcillin-resistant S aureus is incubated at 37ï‚°C on blood agar, one in ten to the power seven organisms expresses nafcillin resistance; while it's miles incubated at 30ï‚°C on agar containing 2–5% sodium chloride, one in 103 organisms expresses nafcillin resistance.

References:

D greenwood, Slack RCB and J Peutherer.Medical microbiology.2001.

JG College, AG Fraser and BP Marmion.Practical Medical microbiology.Fourteenth Edition. Churchill Livingstone, 1996.

JP Micheal, ECS Chan and NR Krieg.Microbiology.Fifth Edition. Delhi: Mcgraw-hill, 1993.

M Cheesbrugh.Medical laboratory manual for tropical countries.London, 2007.

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