Microbial communities are elegantly complex, with all members competing, cooperating, or simply co-existing with all other members. These interactions are especially evident between bacteria and bacteriophages. In many cases, bacteriophages interact with bacterial populations by destroying bacteria through predation [1], by mediating gene transfer through transduction [2], or by controlling gene expression of their bacterial hosts [3]. Some phages integrate into bacterial chromosomes (prophages), which can confer benefits by preventing further phage infection or modulating gene expression, but this can also make the bacteria host susceptible to death by phage induction (process by which DNA damage stimulates excision of the phage genome from the bacterial chromosome, which is followed by phage assembly and release while destroying the bacteria). Some bacteria have developed methods to obtain competitive advantages over prophage containing competitors (lysogenic bacteria) by killing them through stimulation of phage induction, such as those reported by Selva et al [4].
In their report, Selva
et al studied the competitive interactions between
Streptococcus pneumonia (a bacterial pathogen targeted by
pneumococcal vaccine) and
Staphylococcus aureus (an opportunistic pathogen). In their study, they report how relatively low levels of hydrogen peroxide are produced by
S. pneumonia with the intent of stimulating DNA damage in their
S. aureus competitors, thereby promoting phage induction and consequent destruction of the lysogenic bacteria. This is thought to be a particularly effective strategy because
S. aureus bacteria commonly maintain prophages.
In their study, the researchers first showed that lysogenic strains of S. aureus are killed when exposed to levels of hydrogen peroxide often found in S. pneumonia cultures. The non-lysogenic bacteria were not affected by the treatment of hydrogen peroxide, thereby supporting the selective lethality of hydrogen peroxide in lysogenic bacteria. Not only did the group show that lysogenic S. aureus is more susceptible to hydrogen peroxide lethality, but they also showed that phages are produced is greater quantities when the cultures are treated with hydrogen peroxide. S. pneumonia was resistant to the induction effects whether it was lysogenic or not, and co-culture with S. pneumonia and S. aureus resulted in phage induction and destruction of the lysogenic S. aureus strains. The authors made the point that S. aureus produces an effective catalase that protects it from hydrogen peroxide toxicity, suggesting it should be resistant to the effects of hydrogen peroxide. Interestingly, while catalase protects non-lysogenic S. aureus from destruction by hydrogen peroxide, it was significantly less able to protect lysogenic S. aureus from death. The authors speculate that this occurs because significantly more hydrogen peroxide is required to kill a bacterium, compared to that required to stimulate the SOS response. I think this also further emphasizes the potential competitive disadvantage of harboring prophages, because they can make their host susceptible to antimicrobial insults that they would otherwise be resistant to. This overall effect of prophages on antimicrobial susceptibility was also shown to occur upon antibiotic treatment, in which lysogenic bacteria were more susceptible to prophage inducing antibiotics, compared to non-lysogenic bacteria.