Factors affecting the lytic susceptibility of some marine and terrestrial bacteria

Eighteen gram-negative marine bacteria and two terrestrial species, Escherichia coli and Pseudomonas aeruginosa, were examined for their sensitivity to lysis in distilled water after exposure to a salt solution containing a sea water concentration of Mg2+ (0.05 M) or to 0.5 M NaCl. A spectrum of lytic susceptibility was observed among the marine bacteria ranging from those organisms which lysed in distilled water after exposure to the Mg2+-containing solution, through organisms which could be sensitized to lysis by washing with the NaCl solution, to organisms which failed to lyse in distilled water even after having been washed with a solution of 0.5 M NaCl. Pseudomonas aeruginosa and E. coli fell within this spectrum, the former being capable of being induced to lyse in distilled water by washing with 0.5 M NaCl, while the latter failed to lyse in distilled water after this treatment. It was thus concluded that no overall distinction could be made between marine and terrestrial bacteria on the basis of the sensitivity of the two groups of organisms to lysis in freshwater. Quite large decreases in optical density and increases in the release of ultraviolet-absorbing material took place when cells preexposed to the Mg2+-containing solution or to 0.5 M NaCl were subsequently suspended in distilled water even though in some cases no loss of cell numbers could be detected. In most cases two to three times as much K+ as Na+ and 1/10 to 1/100 as much Mg2+ was required to prevent these changes. For three of the marine bacteria and P. aeruginosa grown in a terrestrial type medium little difference in the requirements for Na+ and K+ to prevent the optical density changes was noted. For P. aeruginosa grown in a marine type medium, cells required more K+ than Na+ to prevent these changes.