All naturally occurring isolates of all five species of the marine genus Photobacterium have β-lactamase activity. In this study, the β-lactamase from the laboratory strain P. leiognathi 206 is fully characterised. The enzyme is constitutive and is released from the cell by osmotic shocking techniques, suggesting a periplasmic location. The enzyme is maximally active at pH 6.2 and has over 90% maximum activity at pH 7.0. The hydrolytic activity of the β-lactamase is independent of zinc ion presence. On the basis of substrate profile and inhibition studies the β-lactamase is classified as a Richmond and Sykes Class II enzyme. Crypticity tests indicated that there is no outer membrane permeability barrier to β-lactam substrates. Comparative substrate profiles performed with the β-lactamases from three strains of each species of Photobacterium, indicate that the enzymes from strains of P. leiognathi, P. angustum and P. phosphoreum are active only on penicillins, whereas those from P. fischeri and P. logei also hydrolyze cephalosporins. This division is in agreement with an imminent taxonomic change for the latter two species. Analytical iso-electric focusing of the β-lactamases from 45 Photobacterium strains resulted in pI values which were not necessarily species specific and there was little correlation between the pI of the β-lactamase and its substrate profile, excepting the enzymes from P. angustum and P. logei. Although plasmid DNA is present in many Photobacterium strains, conjugative transfer of β-lactamase activity from six different Photobacterium donors to a restrictionless Escherichia coli mutant was not observed. All attempts to 'cure' the bacteria of β-lactamase activity with five different curing agents, were also unsuccessful. A chromosomal location for the β1a+
gene is postulated.