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.

1. Listeria monocytogenes

It is a small, coccoid, gram positive bacillus, measuring 1-3 mm in length and 0.5 mm width, with a tendency to occur in chains. They are often angled at the point of contact and may resemble diphtheroids or diplococci. Rough forms may be seen as long filaments. It exhibits a characteristic tumbling motility when grown at 25°C, but at 37°C is non-motile.This is because peritrichous flagella are produced at 20-30°C but not at 37°C. They are aerobic, facultatively anaerobic or microaerophilic. Growth can be improved at reduced O₂ tension and with 5-10% CO_2.It grow on ordinary media at 40-42°C, growth improved by the addition of glucose, blood. After 24 hours incubation at 35-37⁰C Colonies are hemolytic. It is catalase positive.

Systematic Bacteriology and Virology