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

Digestion of diverse strains of sedimentary bacteria by deposit feeders occ urs at markedly disparate efficiencies, thus altering the composition of ba cteria in feces and surrounding sediments. In an earlier study, I explored 2 potential mechanisms for resistance to lysis by digestive fluids of Areni cola marina (Polychaeta: Arenicolidae), cell wall ultrastructure and exopol ymeric capsules. This correlative study revealed no relationship between en capsulation and lytic susceptibility across strains, whereas cell wall type (Gram-positive or -negative) was not independent of susceptibility, with G rampositive strains always resistant. In this study, I tested the importanc e 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 thickne ss changes, We found that resistant strains remained resistant regardless o f capsule quantity. Two of 3 strains that normally exhibit some susceptibil ity to lysis, Pseudomonas atlantica and SS-1, however, increased in their s usceptibility with mechanical removal of capsules. Further, capsule quantit y per cell varied throughout the growth cycle for these 2 strains, and lyri c susceptibility varied concomitantly during logarithmic phase, in an inver se manner. Susceptibility of the third susceptible strain, IQ-2, remained u naltered with removal of capsules. This strain also exhibited dramatic chan ge in capsule thickness with culture age, but lytic susceptibility was posi tively correlated to capsule mass per cell, in marked contrast to P. atlant ica and SS-1. Although capsules can reduce the susceptibility of some bacte rial strains, their presence does not convey protection from digestion in a ll strains, nor is encapsulation the only mechanism of resistance. One comm on 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 assemblage s in sediments.