Polyomaviruses

Although relatively little is known about the human polyomaviruses (BK and JC viruses), other animal polyomaviruses e.g. the simian virus SV40 are the best characterized DNA tumor viruses.   Structure :      In comparison with the papillomaviruses, the polyomaviruses are slightly smaller (44 nm diameter).   Pathogenesis :      Polyomaviruses have a high specificity for certain hosts and particular cells within that host. In humans both JC and BK viruses probably enter through the respiratory tract, spread by viremia. In immunocompromised patients, reactivation of virus in the kidneys leads to potentially severe urinary tract infection (BKV) or viremia and central nervous system infection (JCV). Suppressed T-cell function appears to be responsible for reactivation of BK an JC viruses. Both viruses cause tumors when injected into hamsters; however, they are not consistently associated with any human tumors.   Epidemiology :      BK and JC viruses are widely distributed in human populations, as evidenced by the presence of specific antibodies in 70-80% of adults sera.   Clinical Syndromes :      Primary infection is virtually always asymptomatic, although mild respiratory symptoms might occur and cystitis has been reported.      Ureteral stenosis in renal transplant patients appears to be associated with BK virus, as does haemorrhagic cystitis in bone marrow transplant recipients.      Progressive multifocal leukoencephalopathy (PML) is a rare syndrome that occurs in immunocompromised patients, including those with AIDS, and is due to JCV. As the name implies, patients may have multiple neurologic symptoms unattributable to a single anatomic lesion. Impairment of speech, vision, and coordination occurs followed by paralysis of the arms and legs, and finally death.   Laboratory Diagnosis :      Urine cytologic tests can reveal the presence of JC or BK viruses by showing enlarged cells with dense basophilic intranuclear inclusions resembling those induced by cytomegalovirus.      Electron microscopy can be used to visualize viral particles in brain tissue in case of PML.      Culture is the definitive method for documenting active polyomavirus infection, although viral antigen and viral nucleic acid may be demonstrated in tissues or cells by immunofluorescence or immunoperoxidase assays and DNA hybridization techniques respectively.   Treatment, Prevention and Control :      No specific treatment is available, but some stabilization or improvement may occur if the immune suppression can be reduced.

Human Papillomaviruses

Structure and replication :      Human papillomaviruses (HPV) measure 50 nm in diameter. Classification of HPV is based on DNA sequence homology, more than 70 HPV types have been identified.   Pathogenesis :      Papillomaviruses infect and replicate in cutaneous and mucosal epithelium, including epithelial proliferations.      Viral infection remains local and generally regresses spontaneously.      Viral DNA has been found in both benign and malignant tumours.   Epidemiology :      Skin warts, plantar warts, common warts and flat warts are most common in childern and young adults. Genital condylomas are most common among sexually active patients. Laryngeal papillomas are found most commonly in young childern.      HPV infections are transmitted by direct contact. Virus can be found on bathroom floors and towels. Inoculation during sexual intercourse or while passing through an infected birth canal, or by the childhood habits of chewing warts is known to occur.   Clinical Syndromes : 1. Skin warts : Most persons are infected with the common HPV types (1-->4), which infect keratinized surfaces usually on the hands and feet and occur frequently in childhood or early adolescence. 2. Laryngeal warts : Laryngeal papillomas are commonly associated with HPV6 and HPV-11. Multiple laryngeal warts (laryngeal papillomatosis) is usually considered a life-threatening condition in childern because of the danger of airway obstruction. 3. Anogenital warts : Genital warts (condyloma accuminata) occur almost exclusively on the squamous epithelium of the external genitalia and perianal areas and about 90% are also caused by HPV types 6 and 11. 4. Cervical cancer : Neoplastic HPV infection of the genital tract is now recognized as a common sexually transmitted disease. It can envolve in patient with genital warts through grades of cervical dysplasia to invasive cancer.      Infection of the female genital tract by HPV types 16 and 18 and, rarely by other types e.g. 31, 33, 35 & 39 is associated with intraepithelial cervical neoplasia and cancer. The same serotypes are also associated with penile and vulvar cancers as well as laryngeal and oesophageal carcinomas. Coinfection with herpes simplex virus and tobacco smoke are suspected co-factors involved in the progression of high-risk HPV lesions to cancer.   Laboratory Diagnosis :      Molecular probes are the method of choice of establishing the presence of HPV genomes in cervical swabs and in tissue. Human papillomaviruses don't grow in cell cultures, and tests for HPV antibodies are rarely used exept in research surveys.   Treatment, Prevention and Control :      Spontaneous disappearance of warts is the rule, but this may take many months to years so intervention is sometimes warranted, especially for painful or bulky lesions.      Removal by surgical cryotherapy, electrocautery or chemical means can be effective, although recurrences are common.      The best means of prevention is avoidance of direct contact with infected tissue.

Papovaviruses

Papovaviridae includes two genera :      Papillomavirus and Polymavirus. Characteristically these viruses possess circular double stranded DNA genome. They are non-enveloped icosahedral and replicate in the nucleus. The most outstanding characteristics of this group of viruses is their ability to stimulate host cell DNA synthesis and that viral proteins (oncoproteins) interact with cellular tumor suppressor proteins.   ● Human Papillomaviruses. ● Polyomaviruses.

Parvoviruses

Parvoviruses are the smallest DNA animal viruses. They are icosahedral, nonenveloped particles 18-26 nm in diameter. The genome is about 5 kb linear single-stranded DNA with limited coding capacity.      Because of the small coding capacity of their genome, the replication of parvoviruses is dependent on functions supplied by replicating host cells or by coinfecting helper viruses.   Classification :      The family parvoviridae is composed of two subfamilies the parvovirinae which infect vertebrates, and Densovirinae, which infect insects. Parvovirinae comprise three genera: the genus Parvovirus, the genus Erythrovirus and the genus Dependovirus.      Members of both the genus Parvovirus and the genus Erythrovirus are able to replicate autonomously in rapidly dividing cells while the genus dependovirus contains members that are defective and dependent on helper virus (usually an adenovirus) for replication and commonly refered to as adeno-associated viruses. Human adenoassociated viruses have not been linked with any disease.   Parvovirus Infection In Humans :      The most recognised human pathogen in this group of viruses is the parvovirus B19 which is the sole member of Erythrovirus genus. It was formerly named Parvo-virus-like agent (PVLA) strain B19. B19 is a fairly ubiquitous human virus, by the age of 16 years, one-third of the population is seropositive. The immature cells in the erythroid lineage (red cell progenitors) are targets for human B19 parvovirus. Thus the major site of virus replication is the adult marrow and the fetal liver.   Clinical Disease : 1. Erythema infection (fifth disease) is a very mild, febrile, self limiting disease that affects infants. It is associated with a faint rash that has a reticular appearence on the limbs. The face has a "slapped cheek" appearance. 2. Transient aplastic crisis : Parvovirus B19 is the cause of transient aplastic crisis that may complicate chronic hemolytic anaemias e.g. sickle cell anaemia, thalassemia. 3. Infection in immunodeficient host result in persistent infection and cause chronic suppression of the bone marrow. The disease is called pure red cell aplasia. 4. Infection during pregnancy may result in hydrops fetalis and fetal death in 10% of pregnancy metarnal infection before the 20th week of pregnancy due to severe fetal anaemia. There is no evidence that B19 infection causes congenital physical abnormalities.   Laboratory Diagnosis :      The virus is difficult to grow. Diagnostic tests are currently available only in few laboratories : 1. The most sensitive tests detect viral DNA in serum, tissue extracts or fixed tissue by molecular techniques using either nucleic acid probes or PCR. 2. Detection of B19 IgM antibody is indicative of recent infection it is present for 2-3 months after infection. B19 IgG persist for years. Antibodies may not be found in immunodeficient patients with chronic B19 infections.

COAGULASE NEGATIVE STAPHYLOCOCCI

COAGULASE NEGATIVE STAPHYLOCOCCI          These cocci are distinguished from Staphylococcus aureus by their negative reaction in the coagulase test. Being present on the skin, in the mouth and in dust, they are commonly encountered as commensals or contaminants in specimen from skin lesions, wounds, burns, throat, sputum and faeces. The coagulase-negative staphylococci of medical importance are the Staphylococcus epidermidis and Staphylococcus saprophyticus. They are opportunistic pathogens and can cause infection. Staphylococcus epidermidis may cause peritonitis in patients on peritoneal dialysis, chronic septicaemia or endocarditis in patients having heart surgery, bacteraemic infection in patients treated with an indwelling venous catheter left in place for more than 48 hours and septicaemia in immuno-suppressed patients. Also Staphylococcus saprophyticus is an important cause of urinary tract infections in women of child-bearing age.        Novobiocin is used to differentiate between these two species, where Staphylococcus epidermidis is novobiocin sensitive and Staphylococcus saprophyticus is novobiocin resistant.

S T A P H Y L O C O C C I Previous 1   2   3

STAPHYLOCOCCUS AUREUS

STAPHYLOCOCCUS AUREUS     Morphology :      Gram positive cocci about 1µm in diameter, arranged in irregular clusters. They are non-motile, non-spore forming occasionally capsulate.   Cultural Characters :      Staphylococci are facultative anaerobes, grow readily on most bacteriologic media. Optimum temperature for growth is 37°C, but form pigment best at room temperature (20-25°C). Staphylococcus aureus forms grey to golden yellow colonies and produces various degree of haemolysis. On mannitol salt agar; the selective medium for Staphylococci, yellow haloes will surround colonies of Staphylococcus aureus due to acid formation. Non-pathogenic species don't ferment mannitol and the colonies will be pink in colour.   Resistance To Physical And Chemical Agents :      Staphylococci are relatively resistant to dryness, heat (they withstand50°C for 30 minutes) and 10% sodium chloride. They are variably sensitive to many antimicrobial drugs, but are readily killed by certain antiseptics and disinfectants at their correct dilution.     Biochemical Reactions :      Staphylococci produce catalase which differentiates them from streptococci. They ferment many carbohydrates with acid production. Staphylococcus aureus is characterized by the production of coagulase which coagulates plasma, while other staphylococci are coagulase negative. Staphylococcus aureus ferments mannitol with acid production as well as several other sugers and liquefy gelatin.   Antigenic Characters :      Staphylococci contain antigenic polysaccharides (peptidoglycan) and proteins as well as other substances important in cell wall structure.      Protein A is a cell wall component of many Staphylococcus aureus strains, it is antiphagocytic and binds non- specifically to Fc portion of IgG molecule, leaving specific Fab sites free. The subsequent reaction of Fab with homologus (test bacteria) antigen is visualized by clumping of staphylococci. This is the basis of coagglutination method.      Most Staphylococcus aureus strains have coagulase or clumping factor on the cell wall surface, which binds to fibrinogen yielding aggregation of the bacteria.      Some Staphylococcus aureus strains have capsules, which inhibit phagocytosis.   Pathogenesis :      The pathogenic capacity of a given strain of Staphylococcus aureus is the combined effect of its (A) Structural virulence factors (peptidoglycan layer, protein A and capsule, if present). (B) Production of extracellular factors (toxins and enzymes).   I. Toxins : 1. Haemolysins :      An alpha toxin (haemolysin), lyses erythrocytes and damages platelets and vascular smooth muscles. It is dermonecrotic and lethal.      Beta, gamma and delta haemolysins, all lyse human (and other species) erythrocytes, but their role in pathogenicity is less well defined. 2. Leucocidins :      Cause loss of motility and destruction of leucocytes. 3. Enterotoxins : (A-F)      Are important causes of food poisoning, produced when Staphylococcus aureus grows in carbohydrate and protein foods. They are heat stable (resist boiling for 30 minutes) and are resistant to the action of gut enzymes. 4. Exfoliative Toxin :      It causes generalized desquamation of the skin. 5. Toxic Shock Syndrome Toxin-1 : (TSST-1)      Produced by certain strains. It is the same as enterotoxin F.   II. Enzymes : 1. Coagulase :      It is an exo-enzyme, which may be cell bound acting directly on fibrinogen or may be free requiring an accessory factor present in plasma. It coagulates plasma, it deposits fibrin on the surface of staphylococci alerting their ingestion by phagocytic cells or their destruction within such cells. 2. Staphylokinase :      It causes fibrinolysis and clot dissociation and the formation of suppurative micro-emboli responsible for septic metastasis. 3. Hyaluronidase : Spreading factor      It causes hyaluronic acid hydrolysis, which is the fundamental substance forming the connective tissue. 4. β-lactamase : Penicillinase      It inactivates penicillin resulting in resistance to this antibiotic. 5. Deoxyribonuclease :      It hydrolyses DNA.   Clinical Findings :      Staphylococcus aureus is the most important human staphylococcal pathogen. Some of the infections caused by Staphylococcus aureus include : (I) Pyogenic infections : (characterized by production of suppurative and necrotic lesions). 1. Focal : suppuration: folliculitis, impetigo, furuncles, carbuncles, breast abcess, cellulitis, post operative wound infection. 2. Disseminated : with visceral localization and suppuration. From any focus the organism may spread via the lymphatics and blood stream to other parts of the body leading to osteomyelitis, lung abscess, brain abscess, endocarditis, pneumonia and meningitis. (II) Toxin-mediated illness : 1. Food poisoning : due to ingestion of meal containing the enterotoxin previously secreted by Staphylococcus. It is characterized by short incubation period (1-8 hours), nausea, violent vomiting and diarrhoea, followed by rapid convalescence. There is no fever. 2. Toxic shock syndrome : due to TSST-1, there is an abrupt onset of fever, vomiting, diarrhoea, scarletiniform rash, hypotension with cardiac and renal failure in severe cases, usually there is no bacteraemia. 3. Scalded skin syndrome : due to exfoliative toxin causing generalized skin desquamation.   Type 2 diabetes millitus patient has furuncles   Epidemiology :      The nasal carriage of Staphylococcus aureus occurs in 40-50% of humans. Human skin is densely colonized with coagulase-negative species and to a lesser extent with Staphylococcus aureus.      The chief sources of infection are the shedding human lesions, contaminated fomites, human respiratory tract and skin.      Contact spread is important in hospitals, where a large proportion of staff and patients carry antibiotic-resistant staphylococci in the nose or on the skin and this may lead to serious epidemics in newborn nurseries, intensive care units and operating rooms.   Bacteriophage Typing :      Bacteriophage typing is the method of choice adopted by many countries for the epidemiological study of the isolated strains and tracing the source of infection. Strains of Staphylococcus aureus are generally distinguished from one another by their patterns of susceptibility to lysis by an internationally recognized set of 23 standard typing phages.   Detection Of Carriers :      In tracing the source of infection, the suspected patients, hospital staff or food handlers are tested for the possibility of being nasal or skin carriers of strain causing either the hospital epidemic or food poisoning. The isolated strains are then subjected to phage typing.   Laboratory Diagnosis : Specimens :      Pus swab or other material as indicated by type of infection. Smears :      Gram-strained smears often reveal Gram-positive cocci arranged singly, in pairs or clusters among pus cells. Culture :      Specimens are plated on blood agar and MacConkey's agar. In case of staphylococcal food poisoning the specimens (food, vomitus, stools) are plated on mannitol salt agar.      Colonies are identified by their morphology, colour (pigment formation) and presence of haemolysis. The colonies are further identified by their biochemical activities and coagulase production (slide or tube coagulase test).   Treatment : Because of the frequancy of drug-resistant strains, staphylococcal isolates should be tested for antimicrobial susceptibility to help in the choice of the drug. Methicillin-resistant Staphylococcus aureus (MRSA) :      These strains are considered resistant to all beta lactam antibiotics including penicillins, cephalosporins and also non-classic beta lactams such as imipenem. Some strains are also resistant to other drugs including aminoglycosides. The great majority of these strains are sensitive to vancomycin.

S T A P H Y L O C O C C I Previous 1   2   3 Next

STAPHYLOCOCCI

STAPHYLOCOCCI          Staphylococci belong to the family micrococcaceae. They are Gram positive spherical cells usually arranged in grape-like clusters. The genus Staphylococcus has at least 30 species, widely distributed in nature, their normal habitats being the skin and the musous membranes of mammals and birds. The three main species of clinical importance are Staphylococcus aureus, Staphylococcus epidermidis and, Staphylococcus saprophyticus. Staphylococcus aureus is coagulase positive, which differentiates it from other species.   ● Staphylococcus aureus. ● Coagulase Negative Staphylococci (CNS).

S T A P H Y L O C O C C I 1   2   3 Next

Blog Images