Bacteriophage host range and bacterial resistance

Host range describes the breadth of organisms a parasite is capable of infecting, with limits on host range stemming from parasite, host, or environmental characteristics. Parasites can adapt to overcome host or environmental limitations, while hosts can adapt to control the negative impact of parasites. We consider these adaptations as they occur among bacteriophages (phages) and their bacterial hosts, since they are significant to phage use as antibacterials (phage therapy) or to protection of industrial ferments from phage attack. Initially, we address how phage host range can (and should) be defined plus summarize claims of host ranges spanning multiple bacterial genera. Subsequently, we review bacterial mechanisms of phage resistance. These include adsorption resistance, which results in reduced interaction between phage and bacterium; what we describe as "restriction," where bacteria live but phages die; and abortive infections, where both phage and bacterium die. Adsorption resistance includes loss of phage receptor molecules on hosts as well as physical barriers hiding receptor molecules (e.g., capsules). Restriction mechanisms include phage-genome uptake blocks, superinfection immunity, restriction modification, and CRISPR, all of which function postphage adsorption but prior to terminal phage takeover of host metabolism. Standard laboratory selection methods, involving exposure of planktonic bacteria to high phage densities, tend to directly select for these prehost-takeover resistance mechanisms. Alternatively, resistance mechanisms that do not prevent bacterium death are less readily artificially selected. Contrasting especially bacteria mutation to adsorption resistance, these latter mechanisms likely are an underappreciated avenue of bacterial resistance to phage attack.