Heterotrophic bacteria could be divided into saprophytes and parasites. Saprophytes normally do not cause any disease and they survive on dead or decaying materials present in the environment. However, these opportunistic organisms can cause infection when resistance of the host is lowered. Besides, microorganisms are also present normally on the skin, in the upper respiratory tract, the intestines and the lower genitourinary tract of animals and constitute the normal bacterial flora of the body. These are often responsible for production of the disease in animals which are under stress as a result of prolonged antibiotic or steroid administration, nutritional deficiency or sudden change of weather. Parasites could live in animals in any of the following states.
Symbiosis: The parasite lives in the tissues of an animal which will benefit the host in the process of self survival. This state is rare in animals.
Commensalism: The microorganisms live in or on the host without causing any disease. However, this normal flora can turn pathogenic under certain circumstances. These are termed as potential pathogens.
Parasitism: Microbes which would always cause harm to the host when they get access to the host is called the state of parasitism. Such microbes are called obligate pathogen.
The pathogenic organisms possess properties required essentially for their ability to produce the disease. The disease is produced by virtue of one or other or both of the two attributes, viz., invasiveness and toxigenicity. The invasive organisms multiply at the site of infection and generally spread throughout the body. The presence of capsule and certain metabolic products of the organisms protect bacteria from normal defence mechanisms of body. The capacity of certain pathogens such as Pneumococcus to produce disease depends on invasiveness. Pasteurella multocida produces septicaemia in cattle by virtue of invasiveness attributed chiefly due to the presence of capsule on the cell surface.Invasive bacteria may be intracellular i.e. pathogens multiplying in host tissues e.g. Brucella abortus and Mycobacterium bovis, or extra cellular i.e. damaging the host while outside the tissues. Such organisms can’t multiply inside the cells. Pasteurella, Klebsiella species belong to this category.
Several pathogens are able to produce toxins extracellularly which are responsible for the damage to the tissues resulting in the disease. Tetanus is caused by the Clostridium tetani multiplying at a local site in the host and production of toxin. In food poisoning due to Clostridium botulinum, the toxin is produced outside the body in the contaminated food and then consumed to cause the disease. There are a large number of organisms which produce disease by virtue of both these properties in varying proportions. Streptococcus pyogenes, which is an invasive organism also produces toxin. Similarly Staphylococcus aureus and Clostridium perfringens produce toxins which enable the organisms to breach tissue barriers. Many bacteria also produce endotoxins which are different from the extra cellular toxins. Endotoxins are not released by the cells. Endotoxins are also responsible for the disease and death in animals such as caused by Pasteurella multocida.