Medical Microbiology Notes Pdf Free Download
Livia Dossantos
S aureus colonizes the nasal passage and axillae. Sepidermidis is a common human skin commensal. Other species ofstaphylococci are infrequent human commensals. Some are commensals of otheranimals.
S aureus expresses many potential virulence factors. (1) Surfaceproteins that promote colonization of host tissues. (2) Factors that probablyinhibit phagocytosis (capsule, immunoglobulin binding protein A). (3) Toxinsthat damage host tissues and cause disease symptoms. Coagulase-negativestaphylococci are normally less virulent and express fewer virulence factors.S epidermidis readily colonizes implanted devices.
Phagocytosis is the major mechanism for combatting staphylococcal infection.Antibodies are produced which neutralize toxins and promote opsonization. Thecapsule and protein A may interfere with phagocytosis. Biofilm growth onimplants is impervious to phagocytosis.
Infections acquired outside hospitals can usually be treated withpenicillinase-resistant β-lactams. Hospital acquired infection is oftencaused by antibiotic resistant strains and can only be treated withvancomycin.
Multiple antibiotic resistance is increasingly common in Saureus and S epidermidis. Methicillin resistanceis indicative of multiple resistance. Methicillin-resistant Saureus (MRSA) causes outbreaks in hospitals and can beepidemic.
Diagnosis is based on performing tests with colonies. Tests for clumping factor,coagulase, hemolysins and thermostable deoxyribonuclease are routinely used toidentify S aureus. Commercial latex agglutination tests areavailable. Identification of S epidermidis is confirmed bycommercial biotyping kits.
Patients and staff carrying epidemic strains, particularly MRSA, should beisolated. Patients may be given disinfectant baths or treated with a topicalantibiotic to eradicate carriage of MRSA. Infection control programs are used inmost hospitals.
Bacteria in the genus Staphylococcus are pathogens of man and othermammals. Traditionally they were divided into two groups on the basis of theirability to clot blood plasma (the coagulase reaction). The coagulase-positivestaphylococci constitute the most pathogenic species S aureus. Thecoagulase-negative staphylococci (CNS) are now known to comprise over 30 otherspecies. The CNS are common commensals of skin, although some species can causeinfections. It is now obvious that the division of staphylococci into coagulasepositive and negative is artificial and indeed, misleading in some cases. Coagulaseis a marker for S aureus but there is no direct evidence that it isa virulence factor. Also, some natural isolates of S aureus aredefective in coagulase. Nevertheless, the term is still in widespread use amongclinical microbiologists.
S aureus expresses a variety of extracellular proteins andpolysaccharides, some of which are correlated with virulence. Virulence results fromthe combined effect of many factors expressed during infection. Antibodies willneutralize staphylococcal toxins and enzymes, but vaccines are not available. Bothantibiotic treatment and surgical drainage are often necessary to cure abscesses,large boils and wound infections. Staphylococci are common causes of infectionsassociated with indwelling medical devices. These are difficult to treat withantibiotics alone and often require removal of the device. Some strains that infecthospitalized patients are resistant to most of the antibiotics used to treatinfections, vancomycin being the only remaining drug to which resistance has notdeveloped.
DNA-ribosomal RNA (rRNA) hybridization and comparative oligonucleotide analysis of16S rRNA has demonstrated that staphylococci form a coherent group at the genuslevel. This group occurs within the broad Bacillus-Lactobacillus-Streptococcuscluster defining Gram-positive bacteria with a low G + C content ofDNA.
At least 30 species of staphylococci have been recognized by biochemical analysis andin particular by DNA-DNA hybridization. Eleven of these can be isolated from humansas commensals. S aureus (nares) and S epidermidis(nares, skin) are common commensals and also have the greatest pathogenic potential.S saprophyticus (skin, occasionally) is also a common cause ofurinary tract infection. S haemolyticus, S simulans, S cohnii, Swarneri and S lugdunensis can also cause infections inman.
The catalase test is important in distinguishing streptococci (catalase-negative)staphylococci which are catalase positive. The test is performed by flooding anagar slant or broth culture with several drops of 3% hydrogen peroxide.Catalase-positive cultures bubble at once. The test should not be done on bloodagar because blood itself will produce bubbles.
The presence of staphylococci in a lesion might first be suspected afterexamination of a direct Gram stain. However, small numbers of bacteria in bloodpreclude microscopic examination and require culturing first.
The organism is isolated by streaking material from the clinical specimen (orfrom a blood culture) onto solid media such as blood agar, tryptic soy agar orheart infusion agar. Specimens likely to be contaminated with othermicroorganisms can be plated on mannitol salt agar containing 7.5% sodiumchloride, which allows the halo-tolerant staphylococci to grow. Ideally a Gramstain of the colony should be performed and tests made for catalase andcoagulase production, allowing the coagulase-positive S aureusto be identified quickly. Another very useful test for S aureusis the production of thermostable deoxyribonuclease. S aureuscan be confirmed by testing colonies for agglutination with latex particlescoated with immunoglobulin G and fibrinogen which bind protein A and theclumping factor, respectively, on the bacterial cell surface. These areavailable from commercial suppliers (e.g., Staphaurex). The most recent latextest (Pastaurex) incorporates monoclonal antibodies to serotype 5 and 8 capsularpolysaccharide in order to reduce the number of false negatives. (Some recentclinical isolates of S aureus lack production of coagulaseand/or clumping factor, which can make identification difficult.)
The association of S epidermidis (and to a lesser extent ofother coagulase-negative staphylococci) with nosocomial infections associatedwith indwelling devices means that isolation of these bacteria from blood islikely to be important and not due to chance contamination, particularly ifsuccessive blood cultures are positive. Nowadays, identification of Sepidermidis and other species of Staphylococcus isperformed using commercial biotype identification kits, such as API Staph Ident,API Staph-Trac, Vitek GPI Card and Microscan Pos Combo. These comprise preformedstrips containing test substrates.
Because S aureus is a major cause of nosocomial andcommunity-acquired infections, it is necessary to determine the relatedness ofisolates collected during the investigation of an outbreak. Typing systems must bereproducible, discriminatory, and easy to interpret and to use. The traditionalmethod for typing S aureus is phage-typing. This method is based ona phenotypic marker with poor reproducibility. Also, it does not type many isolates(20% in a recent survey at the Center for Disease Control and Prevention), and itrequires maintenance of a large number of phage stocks and propagating strains andconsequently can be performed only by specialist reference laboratories.
Many molecular typing methods have been applied to the epidemiological analysis ofS aureus, in particular, of methicillin-resistant strains(MRSA). Plasmid analysis has been used extensively with success, but suffers thedisadvantage that plasmids can easily be lost and acquired and are thus inherentlyunreliable. Methods designed to recognize restriction fragment length polymorphisms(RFLP) using a variety of gene probes, including rRNA genes (ribotyping), have hadlimited success in the epidemiology of MRSA. In this technique the choice ofrestriction enzyme used to cleave the genomic DNA, as well as the probes, iscrucial. Random primer PCR offers potential for discriminating between strains but asuitable primer has yet to be identified for S aureus. The methodcurrently regarded as the most reliable is pulsed field gel electrophoresis, wheregenomic DNA is cut with a restriction enzyme that generates large fragments of50-700 kb.
S aureus is notorious for causing boils, furuncles, styes, impetigoand other superficial skin infections in humans (Figure 12-1). It may also cause more serious infections, particularly inpersons debilitated by chronic illness, traumatic injury, burns orimmunosuppression. These infections include pneumonia, deep abscesses,osteomyelitis, endocarditis, phlebitis, mastitis and meningitis, and are oftenassociated with hospitalized patients rather than healthy individuals in thecommunity. S aureus and S epidermidis are commoncauses of infections associated with indwelling devices such as joint prostheses,cardiovascular devices and artificial heart valves (Fig. 12-2).
S aureus expresses many cell surface-associated and extracellularproteins that are potential virulence factors. For the majority of diseases causedby this organism, pathogenesis is multifactorial. Thus it is difficult to determineprecisely the role of any given factor. This also reflects the inadequacies of manyanimal models for staphylococcal diseases.
S aureus cells express on their surface proteins that promoteattachment to host proteins such as laminin and fibronectin that form part ofthe extracellular matrix (Figure 12-3).Fibronectin is present on epithelial and endothelial surfaces as well as being acomponent of blood clots. In addition, most strains express a fibrinogen/fibrinbinding protein (the clumping factor) which promotes attachment to blood clotsand traumatized tissue. Most strains of S aureus expressfibronectin and fibrinogen-binding proteins.
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