Traditionally, bacteriostatic antibiotics are agents able to arrest bacterial growth. Despite being traditionally viewed as unable to kill bacterial cells, when they are used clinically the outcome of these drugs is frequently as effective as when a bactericidal drug is used. We explore the dynamics of Escherichia coli after exposure to two ribosome-targeting bacteriostatic antibiotics, chloramphenicol and azithromycin, for thirty days. The results of our experiments provide evidence that bacteria exposed to these drugs replicate, evolve, and generate a sub-population of small colony variants (SCVs) which are resistant to multiple drugs. These SCVs contribute to the evolution of heteroresistance and rapidly revert to a susceptible state once the antibiotic is removed. Stated another way, exposure to bacteriostatic drugs selects for the evolution of heteroresistance in populations previously lacking this trait. More generally, our results question the definition of bacteriostasis as populations exposed to bacteriostatic drugs are replicating despite the lack of net growth.

Bacteriophages are deemed either lytic (virulent) or temperate, respectively depending on whether their genomes are transmitted solely horizontally, or both horizontally and vertically. To elucidate the ecological and evolutionary conditions under which natural selection will favor the evolution and maintenance of lytic or temperate modes of phage replication and transmission, we use a comprehensive mathematical model of the dynamics of temperate and virulent phage in populations of bacteria sensitive and resistant to these viruses. For our numerical analysis of the properties of this model, we use parameters estimated with the temperate bacteriophage Lambda, λ, it’s clear and virulent mutants, and E. coli sensitive and resistant - refractory to these phages. Using batch and serial transfer population dynamic and reconstruction experiments, we test the hypotheses generated from this theoretical analysis. Based on the results of this jointly theoretical and experimental study, we postulate the conditions under which natural selection will favor the evolution and maintenance of lytic and temperate modes of phage replication and transmission. A compelling and novel prediction this in silico, in vitro, and in plastico study makes is lysogenic bacteria from natural populations will be resistant-refractory to the phage for which they are lysogenic as well as lytic phage sharing the same receptors as these temperate viruses.

The most significant difference between bacteriophages functionally and ecologically is whether they are purely lytic (virulent) or temperate. Virulent phages can only be transmitted horizontally by infection, most commonly with the death of their hosts. Temperate phages can also be transmitted horizontally, but upon infection of susceptible bacteria, their genomes can be incorporated into that of their host’s as a prophage and be transmitted vertically in the course of cell division by their lysogenic hosts. From what we know from studies with the temperate phage Lambda and other temperate phages, in laboratory culture, lysogenic bacteria are protected from killing by the phage coded for by their prophage by immunity; where upon infecting lysogens, the free temperate phage coded by their prophage are lost. Why are lysogens not also resistant as well as immune to the phage coded by their prophage since immunity does not confer protection against virulent phages? To address this question, we used a mathematical model and performed experiments with temperate and virulent mutants of the phage Lambda in laboratory culture. Our models predict and experiments confirm that selection would favor the evolution of resistant as well as immune lysogens, particularly if the environment includes virulent phage that share the same receptors as the temperate. To explore the validity and generality of this prediction, we examined ten lysogenic Escherichia coli from natural populations. All ten were capable of forming immune lysogens but their original hosts were resistant to the phage coded by their prophage.

Phenol contamination of soil and water has raised concerns among people living near phenol-producing factories and hazardous waste sites containing the chemical. Phenol, particularly in high concentrations, is an irritating and corrosive substance, making mucosal membranes targets of toxicity in humans. However, few data on the effects of phenol after oral exposure exist. We used an in vitro model employing human intestinal epithelial cells (SK-CO15) cultured on permeable supports to examine effects of phenol on epithelial barrier function. We hypothesized that phenol disrupts epithelial barrier by altering tight junction (TJ) protein expression. The dose-response effect of phenol on epithelial barrier function was determined using transepithelial electrical resistance (TER) and FITC-dextran permeability measurements. We studied phenol-induced changes in cell morphology and expression of several tight junction proteins by immunofluorescence and Western blot analysis. Effects on cell viability were assessed by MTT, Trypan blue, propidium iodide and TUNEL staining. Exposure to phenol resulted in decreased TER and increased paracellular flux of FITC-dextran in a dose-dependent manner. Delocalization of claudin-1 and ZO-1 from TJs to cytosol correlated with the observed increase in permeability after phenol treatment. Additionally, the decrease in TER correlated with changes in the distribution of a membrane raft marker, suggesting phenol-mediated effects on membrane fluidity. Such observations were independent of effects of phenol on cell viability as enhanced permeability occurred at doses of phenol that did not cause cell death. Overall, these findings suggest that phenol may affect transiently the lipid bilayer of the cell membrane, thus destabilizing TJ-containing microdomains.