Role of bacterial exopolymeric capsules in protection from deposit-feeder digestion

Abstract

Digestion of diverse strains of sedimentary bacteria by deposit feeders occurs at markedly disparate efficiencies, thus altering the composition of bacteria in feces and surrounding sediments. In an earlier study, I explored 2 potential mechanisms for resistance to lysis by digestive fluids of Arenicola marina (Polychaeta: Arenicolidae), cell wall ultrastructure and exopolymeric capsules. This correlative study revealed no relationship between encapsulation and lytic susceptibility across strains, whereas cell wall type (Gram-positive or -negative) was not independent of susceptibility, with Gram-positive strains always resistant. In this study, I tested the importance of capsules within strains by comparing susceptibility after manipulation of capsule quantity using 2 techniques: (1) mechanical removal of capsules, and (2) harvest of cells through various growth phases as capsule thickness changes. We found that resistant strains remained resistant regardless of capsule quantity. Two of 3 strains that normally exhibit some susceptibility to lysis, Pseudomonas atlantica and SS-1, however, increased in their susceptibility with mechanical removal of capsules. Further, capsule quantity per cell varied throughout the growth cycle for these 2 strains, and lytic susceptibility varied concomitantly during logarithmic phase, in an inverse manner. Susceptibility of the third susceptible strain, IQ-2, remained unaltered with removal of capsules. This strain also exhibited dramatic change in capsule thickness with culture age, but lytic susceptibility was positively correlated to capsule mass per cell, in marked contrast to P. atlantica and SS-1. Although capsules can reduce the susceptibility of some bacterial strains, their presence does not convey protection from digestion in all strains, nor is encapsulation the only mechanism of resistance. One common feature for all these strains was that susceptibility to lysis declined after cultures reached stationary phase. Consequently, deposit feeding may impact physiological as well as taxonomic structure of bacterial assemblages in sediments.