jMicroorganisms 305 



1. Coccus: spherical; may be found singly (micrococci), in pairs (di- 

 plococci), in clusters (staphylococci) or in chains (streptococci). The 

 cocci cause food poisoning, infectious sore throat, scarlet fever, rheu- 

 matic fever, gonorrhea, pneumonia, and meningitis. 



2. Bacillus: rod-shaped; elongate but oval, short, and thick (cocco- 

 bacilli), attached end to end (streptobacilli). Bacilli cause typhoid fever, 

 cholera, undulant fever, plague, tularemia, whooping cough, anthrax, 

 tetanus, diphtheria, botulism, tuberculosis, and leprosy. Bartonella, a 

 Gram-positive bacillus, causes infectious anemia in dogs, cats, mice, 

 guinea pigs, cattle, sheep, and rats. The organism appears reddish violet 

 in a Giemsa stain and is found in tissue macrophages and erythrocytes. 



3. Spiral forms: curved-rod; a rigid spiral (spirillum) and a flexible 

 spiral (spirochete). These forms can cause relapsing fever, Vincent's 

 disease, syphillis, and yaws. 



Rickettsia are very small. Gram-negative coccobacillary microorgau- 

 isms associated with typhus and spotted fever and related diseases. They 

 may appear as cocci or short bacilli, and may occur singly, in pairs, or 

 in dense masses. They usually are foiurd intracellidarly, seldom living 

 outside of body cells. 



Some species are found only in the cytoplasm, others prefer the nu- 

 cleus. Rickettsia stain best in a Giemsa-type stain or by Machiavello's 

 method. 



Fungi: Actinomycetes and Yeasts. The fungus diseases are described 

 in two gToups. 



1. Superficial mycoses: dermatophytes (ringworm). 



2. Deep-seated mycoses: actinomycosis (cervico-facial, thoracic or ab- 

 dominal infections, restricted mostly to agricidtural workers). 



The actinomycetes are characterized by fine branching filaments 

 (hyphae) that form by intertwining and sometimes anastomosing a 

 colony called a mycelium. Small oval or rod-shaped spores are formed 

 by an aggregation of protoplasm of some of the hyphae. Sometimes the 

 filaments themselves may break up and form bacillary-like bodies which 

 are morphologically indistinguishable from bacteria. These spores are 

 Gram positive and are considered by some as possible transition forms 

 betAveen bacteria and fungi. 



There are t^vo relatively well-defined groups of actinomycetes; one 

 (including all the pathogenic forms) does not form aerial myceliinn, 

 but does show a tendency to segment into bacillary forms. The other 

 group is characterized by formation of spores in aerial hyphae. Some- 

 times the actinomycetes are broken do^vn into the Actinomyces — the 

 anaerobic forms (live best Avithoiu air), and Nocardia — the aerobic 



