Microbial Toxicity Studies

Abstract

Microbial toxins are toxins produced by microorganisms, including bacteria, viruses and fungi. Microbial toxins are important virulence determinants responsible for microbial pathogenicity and/or evasion of the host immune response. Bacterial toxin is a type of toxin that is generated by bacteria. They are classified as either exotoxin or endotoxin. Exotoxins are generated by the bacteria and actively secreted. Endotoxins are part of the bacteria itself. Subtilase cytotoxin (SubAB) is the recently recognised prototype of a new AB5 toxin family secreted by Shiga toxigenic Escherichia coli (STEC). Pasteurella multocida toxin (PMT) is the major pathogenic determinant of Pasteurella multocida. The species P. multocida causes various diseases of animals and humans. The toxin is the causative agent of the economically important atrophic rhinitis in swine. Microorganisms do not form a separate taxonomic group like vertebrates or angiosperms since they are only defined as creatures which are too small to be seen by the naked eye. There are however taxonomic groups like the gram-positive bacteria or the cyanobacteria that contain only microbial species. Microorganisms do not grow more rapidly than plants or animals. The predominant microorganisms in soil, sediments and surface water do not grow rapidly. They have doubling times in the order of magnitude of weeks. Therefore, there is no need to treat microorganisms differently from animals or plants, when performing ecotoxicological risk assessment. The most common way to study the effects of toxic materials on microorganisms is to monitor growth inhibition on agar medium by a viable plate count. Dilutions of natural water samples are plated on agar medium that contains different concentrations of the toxicants and the reduction in colony-forming units (CFUs) relative to control plates containing no toxicant are then observed. The plate count technique is based on the principle that each viable organism will give rise to one colony. This method is simple, economically suitable for statistical analysis and amenable to the examination of large number of water samples. Toxicity of a chemical in an aquatic environment is coupled to its chemical and biological fate. Some very toxic chemicals are innocuous to the environment because their degradation to nontoxic by-products is rapid. Biodegradation is one of the principal fates of these chemicals, and humans depend on the metabolic diversity of microbial populations to ensure that many synthetic organic materials do not accumulate in the environment.