Staphylococci (lat. Staphylococcus) are Gram-positive cocci, 0.8-1.2 micrometers in diameter, which most often appear on clusters on a microscopic specimen.
They can be cultured on normal nutrient media both anaerobically and aerobically (anaerobes are optional). They are immobile and unbroken. Gender Staphylococcus has at least 35 species, and from a medical point of view, the most important is Staphyloccocus aureus.
They are metabolically active, ferment carbohydrates (products lactic acid , but not gases) and produce pigments that vary in color from white to dark yellow. Some are members of the normal flora of the skin and mucous membranes of humans; others cause suppuration, abscess formation, various pyrogens infections , even fatal septicemia. Staphylococcal pathogens usually hemolyze blood and coagulate plasma (see division).
The most common type of food poisoning is caused by heat-stable staphylococcal enterotoxin. Staphylococci rapidly develop resistance to many antimicrobial agents, and present a therapeutic problem. Staphylococci produce a number of enzymes and exotoxins such as coagulase, alpha-toxin, leukocidin, exofoliatins, enterotoxins, and toxic shock toxin (the biochemical nomenclature sometimes surpasses itself), which are responsible for the clinical symptoms of infections with these pathogens.
All staphylococci produce enzyme catalase (catalase-positive), and this distinguishes them from the genus Streptococcus (which are catalase-negative).
Divisions of staphylococci and infections
The genus Staphylococcus is divided into two groups according to the ability to coagulate blood plasma:
- into a Staphylococcus Aureus species that is coagulase-positive, i. coagulates blood plasma;
- into a group of coagulase-negative species, of which about 30 are known today, and are determined by biochemical activity and other physiological properties.
Infections with these pathogens are seen in the form of three types of invasive infections, pure toxicosis, and mixed forms. The antibiotic of choice for the treatment of these infections are penicillinase-resistant penicillins. Laboratory diagnosis includes identification of pathogens by microscopy and culture.
Coagulase-negative staphylococci are classic opportunists. Staphylococcus epidermidis and other species are common agents in foreign body infections due to their ability to form biofilms on the surface of inert objects. Staphylococcus saprophyticus is responsible for approximately 10-20% of acute urinary tract infections in young women.
Staphylococci grow easily on most bacteriological media under aerobic or microaerophilic conditions. They grow fastest at 37 ° C, but produce pigment best at room temperature (20–25 ° C). The colonies are round, smooth, raised and shiny on a solid medium. Staphylococcus aureus usually forms gray to golden yellow colonies. Staphylococcus epidermidis colonies are usually gray to white at primary isolation; many colonies develop pigment only after prolonged incubation. The pigment is not produced under anaerobic conditions or a liquid medium.
Staphylococci contain antigenic polysaccharides and proteins as well as other substances important in the structure of the cell wall. Peptidoglycan (murein), a polysaccharide polymer, forms a solid cell wall exoskeleton. The peptidoglycan is destroyed by strong acids or by exposure to lysozyme. This is important in the pathogenesis of infection: it induces the production of interleukin-1 (which is an endogenous pyrogen) and opsonizing antibodies in monocytes, and these substances can be chemotaxins for polymorphonuclear leukocytes, have endotoxin-like activity, and activate complement.
Teichoic acids (from the Greek τειχος, teichos – wall), which are polymers of glycerol or ribitol phosphate, are bound to peptidoglycans and can be antigens. Antibodies to teichoic acid can be detected by gel diffusion and found in patients with active endocarditis due to S. aureus infection.
Protein A is a cell wall component of many strains of Staphylococcus aureus that binds the Fc portion of IgG molecules other than IgG2. The Fab portion of IgG bound to protein A can bind to a specific antigen.
Protein A has become an important reagent in immunology and laboratory diagnostic technology; for example, protein A with bound IgG molecules directed against a specific bacterial antigen will agglutinate the bacterium containing that antigen (coagglutination).
Some strains of S. aureus have capsules, which inhibit phagocytosis by polymorphonuclear leukocytes, unless specific antibodies are present. Most strains of S. aureus have coagulase and clumping factor on the surface of the cell wall – coagulase binds non-enzymatically to fibrinogen, initiating the aggregation of bacteria.
Serological tests are limited in identifying staphylococci.
Staphylococcal enzymes and toxins
As already mentioned, spatilococci can cause disease both through their ability to multiply and spread throughout tissues, but also through the production of many extracellular substances. Some of these substances are enzymes ; others are considered toxins, although they may also function as enzymes. Many toxins are genetically controlled by plasmids – some may be both chromosomal and extrachromosomal controlled – for others the mechanisms of genetic control are not well known.
- Catalase – Staphylococci create catalase, which converts hydrogen peroxide into water and oxygen. The catalase test distinguishes staphylococci, which are positive, from streptococci, which are negative.
- Coagulase – Staphylococcus aureus produces coagulase, a proenzyme that coagulates oxalized or citrated plasma. Coagulase binds to prothrombin, together they become enzymatically active and begin the polymerization of fibrin. Coagulase can deposit fibrin on the surface of staphylococci, presumably altering their ingestion by phagocytic cells or their destruction within such cells. Catalase production is considered synonymous with invasive pathogenic potential.
- Other enzymes produced by staphylococci include hyaluronidase, a staphylokinase that acts fibrinolytically but acts much more slowly than streptokinase, proteinase, lipase, and beta-lactamase.
- Alpha-toxin is a heterogeneous protein that acts on a wide range of eukaryotic cell membranes. Alpha-toxin is a potent hemolysin. Beta-toxin degrades sphingomyelin and is therefore toxic to many cell types, including human red blood cells.