SUMMARY: The turbidity of suspensions of various Gram-negative bacteria is affected by the tonicity of the suspending medium. The light extinction of the suspension increases in a nearly exponential fashion with the solute concentration and approaches a maximum at concentrations close to 0·15M-NaCl. The maximal optical response to tonicity changes varies widely in different bacterial species, ranging from 30 to 140 % increase in light extinction as compared with the turbidity of the suspension in distilled water. The turbidity changes seem to depend on the viability of the bacterial cells as they cannot be elicited in bacteria killed by various procedures. The influence of the described phenomenon on the determination of bacterial cell concentration by the customary turbidimetric methods is discussed and its application for the estimation of the number of viable cells is suggested.


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