Abstract
In exploring bacterial resistance to bacteriophages, emphasis typically is placed on those mechanisms which completely prevent phage replication. Such resistance can be detected as extensive reductions in phage ability to form plaques, that is, reduced efficiency of plating. Mechanisms include restriction-modification systems, CRISPR/Cas systems, and abortive infection systems. Alternatively, phages may be reduced in their “vigor” when infecting certain bacterial hosts, that is, with phages displaying smaller burst sizes or extended latent periods rather than being outright inactivated. It is well known, as well, that most phages poorly infect bacteria that are less metabolically active. Extracellular polymers such as biofilm matrix material also may at least slow phage penetration to bacterial surfaces. Here I suggest that such “less-robust” mechanisms of resistance to bacteriophages could serve bacteria by slowing phage propagation within bacterial biofilms, that is, delaying phage impact on multiple bacteria rather than necessarily outright preventing such impact. Related bacteria, ones that are relatively near to infected bacteria, e.g., roughly 10+ µm away, consequently may be able to escape from biofilms with greater likelihood via standard dissemination-initiating mechanisms including erosion from biofilm surfaces or seeding dispersal/central hollowing. That is, given localized areas of phage infection, so long as phage spread can be reduced in rate from initial points of contact with susceptible bacteria, then bacterial survival may be enhanced due to bacteria metaphorically “running away” to more phage-free locations. Delay mechanisms—to the extent that they are less specific in terms of what phages are targeted—collectively could represent broader bacterial strategies of phage resistance versus outright phage killing, the latter especially as require specific, evolved molecular recognition of phage presence. The potential for phage delay should be taken into account when developing protocols of phage-mediated biocontrol of biofilm bacteria, e.g., as during phage therapy of chronic bacterial infections.
Highlights
As the total mass of microorganisms is thought to be similar to that of macroscopic organisms [1], microorganisms, given their smaller size, likely are the more numerous
Ones that are relatively near to infected bacteria, e.g., roughly 10+ μm away, may be able to escape from biofilms with greater likelihood via standard dissemination-initiating mechanisms including erosion from biofilm surfaces or seeding dispersal/central hollowing
As our consideration primarily is in terms of resistance to productive, lytic phage infections, this means either that the phage infection is completely blocked in terms of its impact on the host bacterium (100% bacterial survival), is not blocked at all (0% survival save for resistant bacterial mutants), or instead is incompletely blocked
Summary
As the total mass of microorganisms is thought to be similar to that of macroscopic organisms [1], microorganisms, given their smaller size, likely are the more numerous. Here I consider the potential for ―imperfect‖ mechanisms of bacterial resistance to bacteriophages, mechanisms that are unable to outright prevent phage reproduction, to contribute to bacterial survival. This ideally would be sufficient increases in survival to allow for at least some further bacterial reproductive success. (iv) Biofilm-attacking phage strains may not be present across macroscopic environments in overwhelming numbers [8,21,23,24] Building on these ideas, here I consider a number of mechanisms, as summarized, which individually or collectively could serve to slow phage propagation through biofilms, that is, rather than necessarily blocking phage replication altogether. The result is a framework for considering the anti-phage utility of a diversity of biofilm-bacteria structural, physiological, and dispersal strategies as well as challenges which can be associated with phage use as anti-biofilm agents
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