The outer membrane protein profile of Actinobacilus (Haemophilus) pleuropneumonlae grown under iron-restricted and iron-replete conditions was studied by polyacrylamide gel electrophoresis and immunoblotting. A virulent serotype 1 isolate synthesized a novel protein with an apparent molecular weight of 105,000 (105K) and increased the synthesis of a 76K protein under iron-restricted conditions. Both proteins were synthesized within 15 min of establishment of iron-restricted conditions. Proteins of equivalent molecular weights could also be induced by iron restriction in serotype 2, 3, 4, 5, and 7 isolates of A. pleuropneumoniae. Convalescent-phase sera from serotype 1-infected pigs contained antibodies which recognized both the 105K and 76K proteins from all six serotypes examined, indicating that these proteins were expressed in vivo and were immunologically conserved. Cells expressing the 105K and 76K proteins also displayed an enhanced ability to bind Congo red and hemin, suggesting that one or both of these proteins functioned to acquire complexed iron during in vivo growth.

The reduction of exogenous ferric iron by Listeria monocytogenes, a Grampositive food-borne pathogen, was investigated. Using an assay incorporating the ferrous iron chelator ferrozine, we showed that intact cells of L. monocytogenes, when exposed to ferric iron, were able to rapidly reduce and solubilize the iron to the ferrous form. Reduction occurred only after direct contact between the bacteria and the iron source. A number of different ferric iron chelates, including transferrin and lactoferrin-bound iron, haemoglobin, ferritin, and iron complexed to siderophores, could be reduced. The ferric reductase activity was expressed by both reference strains and clinical isolates of L. monocytogenes and by all other species of Lisferia, although significant quantitative differences were observed. In L. monocytogenes, the expression of ferric reductase was not affected by the growth phase of the bacteria nor by the presence or absence of iron in the growth medium. However, expression was greatly reduced in bacteria grown anaerobically and when cultured in media of reduced pH. In addition, bacteria grown at a cold temperature displayed greater ferric reductase activity than cells grown at higher temperatures. A surface-associated ferric reductase system may be one component of a general iron scavenging mechanism which can be used by Listeria growing in a variety of environments.

The outer-membrane protein (OMP) profile of Pasteurella haemolytica grown under iron-replete and iron-restricted conditions was studied by polyacrylamide gel electrophoresis and immunoblotting. A serotype 1 isolate induced the synthesis of a new 77000 M, OMP in ironrestricted media while two other proteins of 100000 M, and 71000 M, were synthesized in increased amounts. None of these proteins were peptidoglycan-associated or heat-modifiable, and only the 100000 M, protein showed some degree of disulphide cross-linking. Kinetic analysis revealed that the iron-repressible proteins appeared in the outer membrane within 15 min of establishment of iron-restricted conditions. Analysis of P. haemolytica isolates representing serotypes 1 to 12 showed that iron-repressible OMPs of 77000 M, and 71 000 M, could be induced in all 12 serotypes but that there was some variability in the expression of the 100000 M, protein. Immunoblotting of OMPs with convalescent sera from P. haemolyticainfected calves indicated that anti bodies directed against all three iron-repressible OMPs were present, suggesting that these proteins were expressed in vivo.

The polymerase chain reaction was used to detect and specifically identify Listeria monocytogenes. A 174-bp region of the listeriolysin 0 gene was shown to be specifically amplified in L. monocytogenes but not in other species of Listeria or in a number of other gram-positive and gram-negative organisms. Less than 50 organisms could routinely be detected by a procedure involving two rounds of 35 amplification cycles each and without the need for subsequent hybridization with labeled probes.

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