Phage-antibiotic synergy: cell filamentation is a key driver of successful phage predation

Phages are promising tools to fight antibiotic-resistant bacteria, and as
for now, phage therapy is essentially performed in combination with
antibiotics. Interestingly, combined treatments including phages and a
wide range of antibiotics lead to an increased bacterial killing, a
phenomenon called phage-antibiotic synergy (PAS), suggesting that
antibiotic-induced changes in bacterial physiology alter the dynamics of
phage propagation. Using single-phage and single-cell techniques, each
step of the lytic cycle of phage HK620 was studied in E. coli cultures
treated with either ciprofloxacin or cephalexin, two
filamentation-inducing antibiotics. In the presence of sublethal doses of
antibiotics, multiple stress tolerance and DNA repair pathways are
triggered following activation of the SOS response. One of the most
notable effects is the inhibition of bacterial division. As a result, a
significant fraction of cells forms filaments that stop dividing but have
higher rates of mutagenesis. Antibiotic-induced filaments become easy
targets for phages due to their enlarged surface areas, as demonstrated by
fluorescence microscopy and flow cytometry techniques. Adsorption,
infection and lysis occur more often in filamentous cells compared to
regular-sized bacteria. In addition, the reduction in bacterial numbers
caused by impaired cell division may account for the faster elimination of
bacteria during PAS. We developed a mathematical model to capture the
interaction between sublethal doses of antibiotics and exposition to
phages. This model shows that the induction of filamentation by sublethal
doses of antibiotics can amplify the replication of phages and therefore
yield PAS. We also use this model to study the consequences of PAS on the
emergence of antibiotic resistance. A significant percentage of
hyper-mutagenic filamentous bacteria are effectively killed by phages due
to their increased susceptibility to infection. As a result, the addition
of even a very low number of bacteriophages produced a strong reduction of
the mutagenesis rate of the entire bacterial population. We confirm this
prediction experimentally using reporters for bacterial DNA repair. Our
work highlights the multiple benefits associated with the combination of
sublethal doses of antibiotics with bacteriophages.