Ecology of Anti-Biofilm Agents II: Bacteriophage Exploitation and Biocontrol of Biofilm Bacteria

Abstract

Bacteriophages are the viruses of bacteria. In the guise of phage therapy they have been used for decades to successfully treat what are probable biofilm-containing chronic bacterial infections. More recently, phage treatment or biocontrol of biofilm bacteria has been brought back to the laboratory for more rigorous assessment as well as towards the use of phages to combat environmental biofilms, ones other than those directly associated with bacterial infections. Considered in a companion article is the inherent ecological utility of bacteriophages versus antibiotics as anti-biofilm agents. Discussed here is a model for phage ecological interaction with bacteria as they may occur across biofilm-containing ecosystems. Specifically, to the extent that individual bacterial types are not highly abundant within biofilm-containing environments, then phage exploitation of those bacteria may represent a "Feast-or-famine" existence in which infection of highly localized concentrations of phage-sensitive bacteria alternate with treacherous searches by the resulting phage progeny virions for new concentrations of phage-sensitive bacteria to infect. An updated synopsis of the literature concerning laboratory testing of phage use to combat bacterial biofilms is then provided along with tips on how "Ecologically" such phage-mediated biofilm control can be modified to more reliably achieve anti-biofilm efficacy.

Keywords: bacteriophage ecology; biocontrol; biofilm control; biofilm eradication; biofilms; ecology; phage ecology; phage therapy.

Figure 1

Ecological scenario of phage interaction with biofilm-associated bacteria, figure as derived in part from that of Abedon [15,17]. Most of the bacteria depicted are shown associated with clonal microcolonies. These in turn are embedded within an “Environment-defining molecular pattern” (EDMP), such as animal-produced mucus, but perhaps also including EPS (EDMP or EPS is shown as shaded boxes, green/bottom for “Start”, red/top for “Finish”). A substantial distance, on the order of millimeters, centimeters, meters, or longer, is assumed to exist between the two shown shaded regions. The scenario begins (1) with the “Initial adsorption” of a bacterial microcolony by a phage, either “Explorer” or “Scout” phage as considered further below. “Settler” phages are those that by chance infect bacteria that are associated with their parental microcolony, i.e., as within the same EPS matrix as the parental infection, and these infections can result (2) in substantial phage reproduction. “Scout” phages are those that by chance infect bacteria associated with neighboring rather than the same microcolonies, that is, as found outside of EPS matrix associated with the parental microcolony. Movement of these phages outside of EDMP or EPS, as depicted, nevertheless does not necessarily occur to a substantial extent, but arrows nonetheless are presented thusly for clarity. This movement (3) can be to adjacent microcolonies (either a smaller gap or instead no gap between microcolony EPS matrices) or (4) to more-distantly located microcolonies, both as found within the same biofilm (larger or multiple gaps). “Explorer” phages escape from their parental biofilm (equivalent to traversing a very large gap between EPS matrices) and, if they are lucky, (5) succeed in (6a) encountering microcolonies or instead (6b) encountering individual, especially planktonic bacteria that are found some distance from the parental focus of infection. Particularly the former (6a) may be found in non-parental biofilms, or alternatively macroscale distances away from the parental locus within the same biofilm, e.g., multiple millimeters or more. No phenotypic or genotypic differences, besides the path of their diffusion and location of subsequent bacterial adsorption, otherwise exist between these different phage “Types”. Within a single biofilm these processes can result in phage plaque-like population growth, or formation of what can be described as a focus of infection. Within microcolonies a progression of phage population growth also can occur, starting with an initial adsorption and then followed by further “Active” phage penetration into the same microcolony. Shown as well (6c) is a potential for some phages to bind to EDMP, such as mucus as described by Barr et al. [49,50]; see also [51]. This binding may allow phages to wait for bacteria to come to them rather than phages diffusing to bacteria, described here as phages “Sitting and Waiting”. These phage virion interactions with molecular patterns may be reversible, allowing for more localized phage searching for bacteria to infect such as within bacterial biofilms (dotted arrow), though such interactions also might locally slow virions during their searches within biofilms for bacteria to infect.