Taxonomy of the Lactobacillus acidophilus Group

A total of 89 strains designated Lactobacillus acidophilus were examined for physiological properties, type of lactic acid produced, cell wall sugar pattern, guanine plus cytosine content of deoxyribonucleic acid (DNA), and DNA homology values compared with selected reference strains. Immunological reactions among a group of the strains were determined by gel diffusion tests, using antiserum to purified lactic acid dehydrogenase (LDH) from a single strain (Sharpe strain A18). Antiserum to glyceraldehyde-3-phosphate dehydrogenase from strain ATCG 4356 was used in microcomplement fixation tests to determine relationships among some strains. DNA preparations from 78 of the 89 strains of L. acidophilus were distributed among six distinct homology groups, designated Al, A2, A3, A4, Bl, and B2. The A group strains had 20 to 30% intergroup homology but very low homology to groups Bl and B2. Likewise, the strains in the two B groups had 20 to 30% intergroup homology but very low homology to the A group strains. Nine strains did not fall into any of the six homology groups. The guanine plus cytosine contents of the DNAs in strains comprising the six homology groups varied from 32 to 38 mol%. In the nine strains not falling into any of the homology groups, the guanine plus cytosine contents were 39 to 47 mol%. Homology group Al, which includes the neotype strain of L. acidophilus (ATCC 4356), is very homogeneous, with most strains showing 95% or more homology to the reference strain. This group corresponds to LDH serogroup III. Strains in the other homology groups showed 60 to 90% homology to then-reference strains. Strains of LDH serogroup II were found in homology groups A2, A3, and A4, and those in LDH serogroup I were in homology groups Bl and B2. In general, the glyceraldehyde-3-phosphate dehydrogenase serology results correlated well with the LDH results. Other phenotypic test results were similar for all of the DNA homology groups. It is recommended that homology group Al be designated L. acidophilus and that strain ATCC 4356 remain the neotype strain.