Evaluation of selective culture media for Lactobacillus, Bifidobacterium and Bacteroides to study the intestinal flora

Abstract number: 1134_01_303

Vael C., De Coninck A., Lammens C., Desager K., Goossens H.

Objectives:  

False positivity rates of selective culture media to study Lactobacillus (Lac) and Bifidobacterium (Bif) in the intestinal flora, range from 9–85% and 29–50% respectively. The aim of our study is to evaluate sensitivity and specificity of established and recently described selective culture media used for enumeration of Lac, Bif and Bacteroides (Bac) from faecal samples.

Methods:  

For Lac 8 selective media (LAMVAB, Sorbitol agar, 2 Rogosa variants and 4 MRS variants) were studied with 24 Lac strains and 14 faecal (non-Lac) strains. For Bif 3 selective media (NPNL, MTPY and RB) were studied with 22 Bif strains and 44 faecal (non-Bif) strains. For Bac 2 selective media (BBE and Schaedler ( VN) were evaluated with 31 faecal (non-Bac) strains. All plates were incubated anaerobically at 37°C for 72 h.

Results:  

The MRS medium, most widely used for enumeration of Lac from faecal samples, is not selective (specificity = 0%) but sustains the growth of all Lac strains tested (sensitivity = 100%). MRS with aerobic incubation improves specificity of the medium (67%) but reduces sensitivity (90%). Similar data were obtained for Rogosa medium: high sensitivity (100%) but low specificity (7%), which was increased to 50% by the addition of metronidazole. LAMVAB medium, recently described by Hartemink et al. (J. Microbiol. Meth., 1999) has the highest specificity (100%) with acceptable sensitivity (83%). The L. acidophilus group is not detected using this medium. NPNL, the reference medium for selective enumeration of Bif has a sensitivity of 100% but a specificity of only 32%. RB medium showed similar results. MPTY medium, recently described by Rada et al. (J. Microbiol. Meth., 2000) demonstrated a sensitivity of 95% and a specificity of 100%. Out of the selective media for Bac, BBE showed the highest specificity (100%) compared to only 48% for Schaedler (VN. BBE detects only the B. fragilis group.

Conclusions:  

Several media used for the selective enumeration of Lac, Bif and Bac allow the growth of faecal contaminants, leading to an insufficient specificity. LAMVAB is the optimal medium for Lac. MTPY is the optimal medium for Bif and BBE is the medium of choice for selective enumeration of Bac.