The Population Biology of Bacterial Plasmids: A PRIORI Conditions for the Existence of Mobilizable Nonconjugative Factors

A mathematical model for the population dynamics of nonconjugative plasmids that can be mobilized by conjugative factors is presented. In the analysis of the properties of this model, primary consideration is given to the conditions under which these nonself-transmissible extrachromosomal elements could become established and would be maintained in bacterial populations. The results of this analysis demonstrate the existence of conditions where, as a consequence of infectious transmission via mobilization, nonconjugative plasmids could become established and be maintained even when the bacteria carrying them have lower reproductive fitnesses than plasmid-free members of the population. However, these existence conditions are stringent and suggest therefore, that it is highly unlikely that plasmids of this type would become established and maintained without some direct selection favoring their carriage. The general implications of these results and limitations of the model are discussed. Brief consideration is also given to the implications of these theoretical findings to the problems of the spread of multiple antibiotic resistance plasmids (R-factors) and the risk of contaminating natural populations of bacteria with chimeric plasmids produced by work with recombinant DNA.     

Temporal orientation in patients with brain disease.

A brief schedule of questions, based on empirically derived norms, disclosed significant inaccuracy in temporal orientation in 24% of a groups of nonaphasic patients with brain disease. Conventional neurological examination detected temporal disorientation in only 15%. Patients with bilateral lesions showed a higher frequency of temporal disorientation than did those with inilateral lesions. In a group of 15 aphasic patients, 5 (33%) showed evidence of temporal disorientation according to the test while only 3 (20%) were detected on clinical examination. It is suggested that a schedule of questions scored on the basis of empirically derived norms should be incorporated into the neurological examination to assess temporal orientation.