Hybridization analysis of the gene encoding a hemolysin (suilysin) of Streptococcus suis type 2: evidence for the absence of the gene in some isolates

A hemolysin gene was cloned from a virulent strain of Streptococcus suis ty pe 2 strain 1933. Analysis of the gene and its product revealed that it is identical to a previously reported hemolysin (suilysin) of S. suis type 2. Southern hybridization analysis of the digested total genomic DNA from S. s uis with the cloned hemolysin DNA sequences as probe indicated that the hem olysin gene is present as a single copy on the genome. Genomic DNA of 63 is olates of S. suis encompassing all known serotypes were examined by DNA hyb ridization and polymerase chain reaction (PCR) studies for the presence of the hemolysin gene homolog. The results of both techniques were identical a nd demonstrated the absence of the hemolysin gene in some isolates. In DNA hybridization studies, three DNA probes derived from the hemolysin encoding gene were used. Results showed that sequences encoding the C-terminal 257 amino acid residues (Probe 1) were the most conserved and hybridized to a 1 .2 kb fragment in 32 (51%) strains and a 4.0 kb fragment in 23 (36%) strain s respectively. Thus, Probe 2 hybridized to the DNA of 55 (87%) of the isol ates tested. The first probe (Probe 1) comprising almost the entire hemolys in gene and the third probe (Probe 3) which consisted of the N-terminal seq uences hybridized only to a 4.0 kb fragment in 23 (36%) of the strains test ed. Eight (13%) of the strains tested were hybridization and PCR negative. The hybridization of the C-terminal end sequences (Probe 2) to the 1.2 kb f ragment in 32 (51%) of the strains and the lack of hybridization of the pro bes to eight (13%) strains may suggest the presence of different types of h emolysin molecule in S. suis strains. (C) 1999 Published by Elsevier Scienc e B.V. All rights reserved.