Rhabdoviruses

The family Rhabdoviridae comprises more than 200 viruses which infect mammals, reptiles, birds, fishes, insects and plants. It has two genera-Vesiculovirus, which causes vesicular stomatitis in horses, cattle and pigs,rarely infects humans & Lyssavirus, which comprises rabies virus. 

Poxviruses

            Poxviruses are the largest viruses that infect vertebrates. This group include the human viruses – Variola (Small Pox), Molluscum contagiosum, animal viruses, bird viruses and insect viruses.

Variola virus is the causative agent of small pox. The vaccinia virus was used as the small pox vaccine. Vaccinia virus is unique in that it is an artificial virus and does not occur in nature as such. It is employed as a vector for the development of recombinant vaccines. The genome can accomodate about 25,000 foreign bps, but it is not suitable as a vector for human use due to its pathogenic effects.

Picornaviruses

            The Picornaviridae family comprises of a large number of very small RNA (pico:small, rna;RNA) viruses with a size of 27-30 nm. They are nonenveloped viruses, resistant to ether and other lipid solvents. Two groups of picornaviruses are of medical importance,  the enteroviruses that parasitise the enteric tract and the rhinoviruses that infect the nasal mucosa. Enteroviruses include Polioviruses type 1, 2, 3, Coxsackie viruses A & B, Enterovirus type 68-72.

Polioviruses causes poliomyelitis, a very ancient disease.

Myxoviridae

            The name Myxovirus came from their ability to adsorb onto mucoprotein receptors on erythrocytes causing haemagglutination. It is classified into two families – orthomyxoviridae consisting of the influenza virus and paramyxoviridae consisting of the Newcastle disease virus, mumps virus, parainfluenza viruses, measles virus and respiratory syncytial virus.

Human Immunodeficiency Virus – AIDS

            The emergence and pandemic spread of the AIDS- Acquired Immunodeficiency Syndrome- have posed the greatest challenge to public health in modern times. It was first recognized in the United States in 1981, as a sudden outbreak of two very rare diseases – Kaposi’s sarcoma and Pneumocystis carinii pneumonia in young adults who were homosexuals or addicted to injected narcotics. They appeared to have lost their immune competence, rendering them vulnerable to fatal infections with relatively avirulent microorganisms, this condition was given the name acquired immunodeficiency syndrome. Isolation of the etiological agent was first reported in 1983 by Luc Montagnier and colleagues from Pasteur Institute, Paris. They isolated a retrovirus from a West African patient with persistent generalized lymphadenopathy and called it lymphadenopathy associated virus (LAV). In 1984 Robert Gallo and colleagues isolated a retrovirus called HTLV-3. two antigenic types of HIV– HIV-1 & HIV-2.

            Virions of the family Retroviridae possess reverse transcriptase enzyme, hence the name. This family has been divided into 3 genera - Retrovirus (HTLV-1 & HTLV-2, oncogenic viruses), Lentivirus (HIV-1 & HIV-2 causes AIDS) and Spumavirus (Human foamy virus).

Herpes Viruses

            The Herpesviridae family contains over a 100 species of enveloped DNA viruses that affects humans and animals. They are characterised by the ability to establish latent infections, enabling the virus to persist indefinitely within infected hosts and to undergo periodic activation.

The herpesvirus capsid is icosahedral, composed of 162 capsomeres, and enclosing the core containing the linear dsDNA genome. The nucleocapsid is surrounded by the lipid envelope derived from the host cell nuclear membrane. The envelope carries surface spikes about 8nm long. Between the envelope and capsid is an amorphous structure called the tegument, containing several proteins and enzymes which aids in replication. The enveloped virion measures about 200nm and the naked virion about 100nm in diameter. Herpesviruses replicate in the host cell nucleus. Like other enveloped viruses, herpesviruses are susceptible to fat solvents like alcohol, ether, chloroform and bile salts. They are heat labile and have to be stored at -70⁰C.

Hepatitis Viruses

            ‘Viral hepatitis’ refers to a primary infection or inflammation of the liver by any one of a heterogenous group of  hepatitis viruses which consists of types A, B, C, D, E and G. Hepatitis viruses are taxonomically unrelated. All the human hepatitis viruses are RNA viruses except for HBV, which is a DNA virus. The features common to them are their hepatotropism and ability to cause a similar icteric illness, ranging in severity.

            The most common viral hepatitis is hepatitis A, formerly called infectious hepatitis, caused by hepatitis A virus (HAV), a single stranded RNA virus usually transmitted by the faecal-oral route. Hepatitis B, formerly called serum hepatitis, is caused by the hepatitis B virus(HBV), a double stranded DNA virus usually transmitted via blood. Hepatitis C, formerly called non-A non-B (NANB) hepatitis. Hepatitis E, (HEV) transmitted by the faecal-route and formerly called non-A non-B non-C hepatitis. An especially severe form of the disease hepatitis D or delta hepatitis, is caused by the presence of both hepatitis D virus (HDV) and HBV.

Arboviruses

            Arboviruses are arthropod-borne viruses, viruses of vertebrates biologically transmitted by blood-sucking insects. They multiply in them, transmitted by bite to vertebrate hosts. Arboviruses  have a very wide host range including many species of animals and birds. The ability to multiply in arthropods is their special characteristic. The most important arbovirus vectors are mosquitoes and ticks. In the laboratory, mice are commonly used for growing arboviruses. They can be grown in the yolk sac or chorioallantoic membrane of chick embryo, in tissue cultures of primary cells like chick embryo fibroblasts, HeLa cells. Most arboviruses agglutinate the red cells of one-day old chicks. Hemagglutination is influenced by pH , temperature etc. Arboviruses are labile, inactivated at room temperature, bile salts, ether.

Adenoviruses

                        Adenoviruses are a group of medium sized, non enveloped, ds DNA viruses that share a common complement fixing antigen. They infect humans, animals and birds, shows strict host specifity. At least 47 serotypes of adenoviruses have been isolated from human sources. Adenovirus infections are common worldwide mostly in children. Many infections are asymptomatic. The virus may persist in the host for many months.It cause infections of the respiratory tract and eyes. These viruses carry DNA up to 7kb and are being investigated as potential vectors in gene therapy.

Spirochaetes

        Elongated, motile, flexible bacteria twisted spirally along the long axis. A charecteristic feature is the presence of endoflagella-polar flagella, situated between outer membrane and cell wall. They are Gram negative of varying sizes. Many are free living saprophytes, while some are obligate parasites. They may be aerobic, anaerobic, facultative. Reproduction by transverse fission. Human pathogens are: Treponema, Borrelia, Leptospira. The spiral shape and serpentine motility of the spirochaetes depend upon the integrity of these endoflagella. Motility is of 3 types : (i) flexion & extension (ii) cork-screw like rotary movement (iii) translatory motion.

Propionibacterium

            Gram positive rod shaped, causes acne.It is most often the result of male sex hormones that stimulates sebaceous glands to increase insize and secrete more sebum. It occurs in both sexes, because the hormones are produced by the adrenal glands as well as by the testes. Microorganisms feed on sebum, and ducts of the glands and  surrounding tissues become inflamed.

Mycobacterium

            Mycobacteria are slender rods, sometimes show branching filamentous forms. In liquid cultures they form a mold-like pellicle, hence the name mycobacteria, meaning fungus-like bacteria. They do not stain readily, but once stained resist decolurisation with dilute mineral acids, therfore they are called “Acid Fast Bacilli” or AFB. They are aerobic, nonmotile, noncapsulated, nonsporing. Generally slow growth, obligate parasites, opportunistic pathogens and saprophytes.

Haemophilus

            Small, non-motile, non-sporing, Gram-negative bacilli that are parasitic on human beings and animals.  They are characterized by their requirement of one or both of two accessory growth factors present in blood, ie., X and V.  It was isolated in 1892 by Pfeiffer, hence known as Pfeiffer's bacillus. The genus name is derived from the Greekwords meaning 'blood loving'. H. influenzae is an exclusively human pathogen whose complete genome has sequenced. The important  species are   Haemophilus influenzae, H. aegyptius, H. ducreyi

Corynebacterium

            Corynebacteria are Gram positive, non-acid fast, non-motile rods. They frequently show club-shaped swellings and hence the name, ‘coryne = club’. The most important genus is C. diphtheriae, causative agent of diphtheria.

Chlamydiae

Chlamydia are obligate intracellular parasite of humans, animals and birds. Based on human diseases, they were called psittacosis – lymphogranuloma - trachoma, PLT viruses agents. They lack enzymes of the electron transport chain and so require ATP and nutrient sources from the host cells. Therefore,they are called energy parasites.

Actinomycetes

            Like fungi they form a mycelial network of branching filaments, like bacteria they are thin, possess cell walls containing muramic acid, prokaryotic nuclei. So actinomycetes are considered as transitional forms between bacteria and fungi. They are Gram positive, nonmotile, non sporing, non capsulated filaments that break up into bacillary and coccoid elements. Most are free living, particularly in soil. Human pathogenic actinomycetes belong to four genera : Actinomyces, Nocardia, Streptomyces & Actinomadura. Actinomyces is non acid fast and anaerobic or microaerophilic,  Nocardiais aerobe and acid fast,  Streptomyces& Actinomadura are aerobes and non acid fast. Some species of Streptomyces may cause disease rarely, but their importance is as the major source of antibiotics.

Streptococcus

            Gram positive, catalase negative, nonsporing, spherical or ovoid cells. Cell division occurs in one plane, therefore, they are arranged in pairs or chains. Most strains produce capsules composed of hyaluronic acid. Generally nonmotile. Majority are aerobes and facultative anaerobes, some are obligate anaerobes, some require CO₂ for growth. Also growth can be enhanced by the addition of glucose, blood or serum. Their G + C content is 30 – 46 mol%. Streptococci form normal flora of man and animals. They inhabit respiratory tract, live harmlessly as commensals.

Important disease causing species are : Streptococcus pneumoniae

                                                          Streptococcus pyogenes

                                                         Streptococcus mutans

Staphylococcus

The family Micrococcaceae is composed of three genera : Micrococcus, Planococcus and Staphylococcus. They are catalase positive, gram positive, spherical cocci which divide incompletely in three perpendicular planes to form pairs, tetrads, short chains. Micrococci are coagulase negative and usually oxidase positive, are rarely associated with infections. They have a tendency to produce a yellow pigmented colony. Planococci are capable of withstanding saline concentrations of upto12%, arranged in tetrads, produce a yellow – brown pigment on nutrient agar. Staphylococci are associated with colonisation and/or infection of man. Some are coagulase negative while some are positive. They are ubiquitous, cause localized lesions. Snce they develop resistance to penicillin and other antibiotics they are important as human pathogen, especially in the hospital environment.

Staphylococci were first seen in pus by Koch in 1878, first cultivated in the medium by Pasteur in 1880 and named by Alexander Ogston in 1881.

Important species are:

                                    S. aureus

                                    S. epidermidis

Bacillus

Gram positive, motile, spores found in soil, water, air and common contaminants of bacteriological media.Two important pathogenic species are B.anthracis and B.cereus. Genus Bacillus has numerous applications such as:

·         Filter paper strips impregnated with spores of B. subtilis have been used to test the efficacy of hot air oven, and B. staerothermophilus to test the autoclave and low-temperature steam-formaldehyde sterilizer. B. globigi used to test EtO sterilizer, B. pumilis to test the efficacy of ionising radiation.

·         These produce antibiotics such as bacitracin, polymyxin and gramicidin and act as biological controls in assays of folic acid, aflatoxins and hexachlorophane.

·         B. thuringiensis has  been widely used as an insecticide mainly for pest  control  on food crops.

·         B. subtilis and other species have provided a model for microbial genetics.

The resistance of spores of Bacillus species to drying, UV, heat and chemical disinfectants make these organisms troublesome contaminants of food leading to a variety of food spoilage defect and pharmaceutical products. Their absence from these products acts as a good indicator of cleanliness of the product.

Mycoplasmas

           Mycoplasmas are the smallest (0.2-0.3mm) free-living microorganisms, therefore they can pass through bacterial filters, and one of the most pleomorphic, since they are devoid of cell walls. They occur as granules and filaments of various sizes. The granules may range from 100-1000 mm in size, with coccoid, balloon,disc, ring or star forms. The filaments are slender of varying lengths and show branching. Multiplication is by binary fission, budding or chain of beads produced. A distinctive feature seen in some species is a bulbons enlargement with a differentiated tip structure, by means of which the organisms get attached to suitable host cells carrying neuraminic acid receptors. They may be responsible for the hemadsorption shown by some species. They do not possess spores, flagella or fimbria. Some species exhibit a gliding motility. Mycoplasmas are Gram negative, but better stained by Giemsa stain. The cells are bounded by a soft trilaminar unit membrane containing sterols. Because of their plasticity, they can pass through filters and often mistaken for viruses.