The term Staphylococcus is derived from the Greek term staphyl, meaning “a bunch of grapes.” This name refers to the fact that the cells of these gram-positive cocci grow in a pattern resembling a cluster of grapes; however, organisms in clinical material may also appear as single cells, pairs, or short chains. Most staphylococci are 0.5 to 1 µm in diameter and are nonmotile, aerobic or facultative anaerobic, and catalase-positive and grow in a medium containing 10% sodium chloride and at a temperature ranging from 18ºC to 40ºC.
The organisms are present on the skin and mucous membranes of humans and other mammals, and birds. Staphylococcus is an important pathogen in humans, causing a wide spectrum of life-threatening systemic diseases; infections of the skin, soft tissues, bones, and urinary tract; and opportunistic infections. The species most commonly associated with human diseases are S.aureus (the most virulent and best-known member of the genus), S.epidermidis, S.saprophyticus, and S.haemolyticus.
S. aureus colonies are golden as the result of the carotenoid pigments that are formed during their growth, hence the species are called so. It is also the only species found in humans that produces enzyme coagulase; thus, all other species are commonly referred to as coagulase-negative staphylococci.
Staphylococcus aureus
S. aureus is ubiquitous, existing everywhere in nature. It constitutes part of the normal flora of the skin, nose, throat, gastrointestinal tract, and genital tract of 25-50% of humans and animals. S.aureus is among the most resistant of the nonspore formers to adverse enviromental conditions and physical and chemical agents. The organism can survive for as long as 14 weeks in dried pus and is killed by 70% ethanol only after a 10-min contact period. Staphylococci produce diseases because of their ability to spread in tissues and form abscesses, produce extracellular enzymes or exotoxins and combat host defences.
Cell wall virulence factor of Staphylococcus aureus
1. Capsule. A loose-fitting, polysaccharide layer (slime layer) protects bacteria by inhibiting the chemotaxix and phagocytosis. It is also facilitate the adherence of bacteria to catheters and other synthetic material.
2. Teichoic acids are bound convalently to the peptidoglycan. It mediates the attachment of staphylococci to mucosal surfaces through their specific binding to fibronectin.
3. Protein A is a cell wall component also covalently linked to peptidoglycan and with the ability to bind to the Fc portion of Ig G and extracellular matrix glycoprotein. Protein A may contribute to adherence and possess antiphagocytic activity.
Toxins
1. α, ß, δ, and γ toxins (produced by most strains of S. aureus) attack mammalian cell (including red blood cell) membranes, and are often referred to as haemolysins. α toxin exhibits dermonecrotic activity that contributes to tissue necrosis.
2. Toxic shock syndrome toxin-1 (TSST-1) is a protein produced by virtually all strains of S. aureus and responsible for the clinical manifestations of toxic shock syndrome.
3. Enterotoxin A, B, and D molecules are heat-stable proteins capable of withstanding boiling for 30 min and produced by 30% to 50% of all S. aureus strains. Synthesis is plasmid or chromosomally mediated. Enterotoxin A and D are responsible for staphylococcal food poisoning by inhibiting water absorption from the intestinal lumen and inducing diarrhea. Enterotoxin B damages the intestinal epithelium and produces colitis.
4. Exfolitative (epidermolytic) exotoxin is produced by some (5% to 10%) strains of S. aureus. Synthesis is plasmid or chromosomally mediated. By causing intraepidermal splitting of tissues and necrosis, it is responsible for the clinical manifestations seen in scalded skin syndrome (SSS).
Enzymes
1. Lipases are lipid-hydrolyzing enzymes, which allow the organisms to invade cutaneous and subcutaneous tissues by splitting fats and oils accumulating on the skin. All strains of S. aureus and more than 30% of the strains of coagulase-negative staphylococci produce several different lipases.
2. Leucocidin is an exotoxin that contributes to the survival of the organism, it destroys polymorphonuclear leukocytes.
3. Coagulase is an enzyme produced by the organism during its growth. The role of coagulase in the pathogenesis is speculative, but coagulase may cause the formation of fibrin layer around a staphylococcal abscess, thus localizing the infection and protecting the organisms from phagocytosis.
4. Hyaluronidase is produced by over 90% of S. aureus strains. It is an enzyme that hydrolyzes the hyaluronic acid constituent of connective tissue ground substances and thus facilitates the spread of the organism through the tissues.
5. Staphylokinase (fibrinolysin) is produced by virtually all S. aureus strains. It dissolves fibrin clots and thus contributes to the spread of the organism from local sites.
6. Nuclease is another enzyme for S. aureus. The role of this enzyme in the pathogenesis of infection is unknown.
7. Penicillinase. More than 90% of staphylococcal isolates were susceptible to penicillin in 1941, the year the antibiotic was first used clinically. Resistance to penicillin quickly developed, however, primarily because the organisms could produce penicillinase (ß-lactamase).
8. Catalase. All staphylococci produce catalase, which catalyzes the conversion of toxic hydrogen peroxide to water and oxygen. Hydrogen peroxide can accumulate during bacterial metabolism or after phagocytosis.
Superantigens: enterotoxins and toxic shock syndrome toxin
S. aureus can express two different types of toxin with superantigen activity, enterotoxins, of which there are six serotypes (A, B, C, D, E and G) and toxic shock syndrome toxin (TSST-1). Enterotoxins cause diarrhea and vomiting when ingested and are responsible for staphylococcal food poisoning. When expressed systemically, enterotoxins can cause toxic shock syndrome (TSS) - indeed enterotoxins B and C cause 50% of non-menstrual TSS. TSST-1 is very weakly related to enterotoxins and does not have emetic activity. TSST-1 is responsible for 75% of TSS, including all menstrual cases. TSS can occur as a sequel to any staphylococcal infection if an enterotoxin or TSST-1 is released systemically and the host lacks appropriate neutralizing antibodies. Tampon-associated TSS is not a true infection, being caused by growth of S aureus in a tampon and absorption of the toxin into the blood stream. TSS came to prominence with the introduction of super-absorbent tampons; and although the number of such cases has decreased dramatically, they still occur despite withdrawal of certain types of tampons from the market.
Superantigens stimulate T cells non-specifically without normal antigenic recognition. Up to one in five T cells may be activated, whereas only 1 in 10,000 are stimulated during antigen presentation. Cytokines are released in large amounts, causing the symptoms of TSS. Superantigens bind directly to class II major histocompatibility complexes of antigen-presenting cells outside the conventional antigen-binding grove. This complex recognizes only the Vb element of the T cell receptor. Thus, any T cell with the appropriate Vb element can be stimulated, whereas normally antigen specificity is also required in binding.
Diseases of the S. aureus
1. Localized skin infections include impetigo, folliculitis, furuncles, and carbuncles. Impetigo, a superficial infection affecting mostly young children, occurs primarily on the face and limbs. Initially, a small macule (flattened red spot) is seen, and then a pus-filled vesicle (pustule) on an erythematous base develops. Folliculitis is a pyogenic infection in the hair follicles. The base of the follicle is raised and reddened, and there is a small collection of pus beneath the epidermal surface. If this occurs at the base of the eyelid, it is called a stye. Furuncles (boils), an externsion of folliculitis, large, painful, raised nodules with an underlying collection of dead and necrotic tissue. Carbuncles occur when furuncles coalesce and extend to the deeper subcutaneous tissue. Multiple sinus tracts are usually present.
2. Scaled Skin Syndrome is a disease that occurs in infants and children 4 years of age or under. In this syndrome, the organisms release exfolitative toxin, which is resposible for the extensive intraepidermal splitting and bullous necrosis of the tissue.
3. Toxic Shock Syndrome is a disease that was initially described in children, although it is now recognized as primarily a disease in menstruating women. At present, 80% to 90% of patients with TSS are menstruating women. Highly absorbent tampons contribute to the initiation of the disease by providing a favarable enviroment for the growth of resident S.aureus. The disease is initiated with the localized growth of toxin-producing strains of S.aureus in the vagina or a wound, followed by release of the toxin into the blood stream. Clinical manifestations start abruptly and include fever, hypotension, and a diffuse macular erythematous rash.
4. Food Poisoning is one of the most common food poisoning in the world. The organisms are usually introduced into food, such as processed meat, pastries, potato salad and ice cream. The contaminated food is kept at room temperature, during that time the organisms multiply and release heat stable enterotoxin A or D. Following ingestion of the food, the onset of disease is abrupt and rapid with an incubation period of only 1-6 hs. Staphylococcal food poisoning is characterized by severe vomiting, diarrhea, and abdominal pain. The absence of fever is an important observation in the differential diagnosis of staphylococcal food poisoning.
5. Colitis is a disease that is observed in patients whose normal bowel is altered by the oral administration of broad-spectrum antibiotics that selectively permit overgrowth by antibiotic resistant, enterotoxin B producing strains of S.aureus. Enterotoxin B damages the intestinal epithelium and produces fever, diarrhea, and abdominal cramps.
6. Pneumoniae is a disease that occurs among immnosuppressed patients, the aged, infants less than one year of age, and frequently in children with measles and influenza.
7. Other diseases like osteomyelitis, septicemia, and septic arthritis.
Principle of the Laboratory Diagnostics
Specimens obtained depend on the disease process and include lesion material, pus, sputum, blood, spinal fluid, and feces.
İsolation and identification of S. aureus requires initial cultivation on blood agar and/or specific medium and overnight incubation under aerobic conditions at 37ºC. The organism may be identified as a gram-positive, catalase positive coccus exhibiting coagulase.
Resistance of Staphylococci to Antimicrobial Drugs
Hospital strains of S. aureus are often resistant to many different antibiotics. Indeed, strains resistant to all clinically useful drugs, apart from the glycopeptides vancomycin and teicoplanin, have been described. The term MRSA refers to methicillin resistance and most methicillin-resistant strains are multiply resistant, too. Plasmid-associated vancomycin resistance has been detected in some enterococci and the resistance determinant has been transferred from enterococci to S. aureus in the laboratory and may occur naturally. S. epidermidis nosocomial isolates are also often resistant to several antibiotics including methicillin. In addition, S. aureus expresses resistance to antiseptics and disinfectants, such as quaternary ammonium compounds, which may aid its survival in the hospital environment.
Since the beginning of the antibiotic therapy S. aureus has responded to the introduction of new drugs by rapidly acquiring resistance by a variety of genetic mechanisms including (1) acquisition of extrachromosomal plasmids or additional genetic information in the chromosome via transposons or other types of DNA insertion and (2) by mutations in chromosomal genes.
Many plasmid-encoded determinants have recently become inserted into the chromosome at a site associated with the methicillin resistance determinant. There may be an advantage to the organism having resistance determinants in the chromosome because they will be more stable.There are essentially four mechanisms of resistance to antibiotics in bacteria: (1) enzymatic inactivation of the drug, (2) alterations to the drug target to prevent binding, (3) accelerated drug efflux to prevent toxic concentrations accumulating in the cell, and (4) a by-pass mechanism whereby an alternative drug-resistant version of the target is expressed.
Treatment
The antibiotics of choice are oxacillin (or other penicillinase-resistant penicillin) or vancomycin for oxacillin-resistant strains.
The focus of infection (e.g., abscess) must be identified and drained. Treatment is symptomatic for patients with food poisoning
Coagulase-Negative Staphylococci
A. S. epidermidis is present in large numbers as part of the normal flora of the skin. As such it is frequently recovered from blood cultures, generally as a contaminant from the skin. Despite its low virulence, it is a common cause of infection of implants such as heart vulves and catheters. Cell envelope factors that facilitate attachment to plastic surfaces act as virulent factors. Acquired drug resistance by S. epidermidis is even more frequent than by S. aureus.
B. S. saprophyticus is a frequent cause of cystitis in women, probably related to it in occurrence as part of normal vaginal flora. It tends to be sensitive to most antibiotics, even penicillin G. S. saprophyticus can be distinguished from S. epidermidis and most other coagulase-negative staphylococci by its natural resistance to novobiocin.
