Functional Properties of Plasmids as Related to Enumeration of Bacteria Important to the Food Microbiologist

Abstract

Plasmids are extrachromosomal elements that behave like auxiliary chromosomes and contain the basic structure of the replicating unit. They were initially recognized by the unusual phenotypic characteristics they confer upon a cell, such as ability to promote genetic transfer by conjugation; resistance to antibiotics and metal ions; production of bacteriocins, toxins, antigens; and other factors. Much of the work on plasmids and plasmid-mediated characteristics has been conducted with various genera and species of the family Enterobacteriaceae. Present data indicate that although plasmid transfer occurs with variable frequency, intergeneric transfer within the family is invariably successful. Although a great deal of investigation has been focused on plasmid-mediated resistance to antibiotics, many workers have determined that a number of plasmids control other phenotypic characters. For example such plasmids appear to mediate production of bacterial products and/or enzymes which may confer a selective ecological advantage to bacteria in their inter-relationships with their hosts and other microorganisms. The isolation of these phenotypically altered microorganisms by food, medical, and public health microbiologists may signify an important ubiquitous phenomenon. There is increasing evidence that selective forces operate in both the laboratory and in nature which should alert the microbiologist that any of the present-day identification schema must be utilized with some caution. To this end technical advances in identification of microorganisms by use of a large battery of biochemical tests has increased the accuracy of identification and measurably reduced the misidentification of atypical bacteria. Availability of such data may assist the microbiologist to establish baseline information essential for development of prospective studies for defining the reservoirs of phenotypically altered microorganisms and their pathogenic potential for the general population. The full importance of plasmid-mediated characteristics in bacterial populations in food products and the role they will play in the future is largely unknown. The origin and selective advantage that strains harboring plasmids in the natural ecology is also unknown. Detailed knowledge of microorganisms maintaining plasmids may be important economically, since the emergence of a number of antibiotic-resistant microorganisms may have contributed a significant role in development of disease among domestic animals because oft he manner in which animal husbandry is practiced in many countries. Extrapolated, one could speculate about an ever-increasing reservoir of similar bacterial species, potentially transmissible to man. Because the factors which control mobilization of plasmids and the important ecological selective advantage conferred upon bacterial populations by plasmids, it is essential that we as microbiologists become cognizant of their existence and to fully understand the mechanisms of perpetuation of plasmid-mediated characteristics.