The biochemical, immunological, and biological properties of an S layer purified from an strain (AH-342) involved in a case of bacteraemia were investigated. The S layer selectively removed from the cell surface was composed of a single acidic (pI 4·65) protein subunit (surface array protein, SAP) with a molecular mass of approximately 52 kDa. Amino acid analysis of this 52 kDa protein indicated a molecule composed of 498 amino acids with 46% hydrophobic residues. No cysteine residues were detected. The first 35 residues of the N-terminus were sequenced by Edman degradation; only 4–24% homology was noted between this sequence and those previously published for SAPs of (A450) and a strain of (TF7) originally isolated from a moribund fish. Polyclonal antibodies raised against AH-342 SAP were genospecific, reacting only against S layers produced by strains and not those from . Acute serum from the bacteraemic patient from whom AH-342 was isolated reacted strongly with the SAP of AH-342 in immunoblot studies. Purified SAP, when intraperitoneally co-inoculated with SAP strains of into Swiss-Webster mice, could reduce the 50% lethal dose by approximately 30–70 fold. The results suggest that the SAP of strains may play an important role in systemic dissemination after invasion through the gastrointestinal mucosa.


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