2 CRL1289, Lacidophilus CRL1266 (H202-generating strains), L gasseri CRL1259 (organic acid producer), L johnsonii CRL1294 (aggregating), and L salivarius CRL1328 (bacteri- ocin producer). They have been isolated from the human vagina of women from Tucuma´n, Argentina, and identiﬁed by biochemical proﬁles, sugar fermentation patterns, and API 50 system (BioMe´rieux Vitec, Inc, France) [10]. NC- CLS type strain, Staphylococcus aureus ATCC29213 from the American Type Culture Collection, was employed as refer- ence strain. All the microorganisms were stored in milk-yeast extract at −70◦C. Prior to the assays, they were subcultured twice in LAPTg broth [16], and a third time in the media where the susceptibility to antibiotics assay was going to be performed: MRS [17], LAPTg, or Mu¨ller Hinton broth. Antimicrobial agents Inhibitors of the cell wall synthesis (oxacillin, aminopenicil- lins, ceftazidime, ceftriaxone, cefotaxime, imipenem, aztre- onam, and vancomycin), protein synthesis (kanamycin, gen- tamicin, streptomycin, tetracyclines, chloramphenicol, clar- ithromycin, erythromycin, and nitrofurantoin), and nucleic acid synthesis (trimethoprim-sulfamethoxazole, rifampin; norﬂoxacin, ciproﬂoxacin, nalidixic acid, pipemidic acid, and metronidazole) were employed for inhibition tests. They were used as commercial discs (Britania, Argentina) or prepared from drugs provided by diﬀerent companies (Sigma, USA; Merck, Germany; Britania, Argentina; ICN, Argentina). Disc diffusion method Antimicrobial susceptibility was studied by employing the method described by Bauer et al [18] for clinical isolates, modiﬁed by using three diﬀerent base agar media: Mu¨ller Hinton, LAPTg, and MRS agars. Frozen microorganisms were subcultured twice in LAPTg broth and a third time in MRS, LAPTg, or Mu¨ller Hinton broth for 14 hours at 37◦C. Suspensions were adjusted to tube 5 in McFarland scale (108 CFU/mL) and the microorganisms were (a) dis- seminated on the surface of MRS, LAPTg, or Mu¨ller Hin- ton agar plates with embedded swabs and (b) included into the agar. To include the lactobacilli into the agar, 100 µL of the microbial suspension were mixed with 12 mL of melted agar (melted and cooled down to 45◦C) and then poured on plates. Antibiotic discs were placed on the surface of the agar (six discs in each plate) and the plates were incubated for 24 to 48 hours at 37◦C under microaerophilic conditions. After the incubation, the diameter of the halos was mea- sured. Minimal inhibitory concentrations The MICs were determined in LAPTg broth and agar. Solu- tions of each antibiotic at concentrations of 10 to 50 mg/mL were prepared. They were serially diluted in LAPTg broth and added to LAPTg broth or 45◦C melted agar to obtain Infectious Diseases in Obstetrics and Gynecology Figure 1: Semiquantitative disc assay developed in LAPTg agar for Lactobacillus acidophilus CRL1251 inoculated on the surface. ﬁnal concentrations of 1 to 1000 µg/mL. Fifty µL of exponen- tial growth phase microorganisms at concentration of 107 to 108 CFU/mL were inoculated in LAPTg with antibiotics. Cultures were incubated up to 48 hours at 37◦C and the in- hibition of growth was spectrophotometrically determined at 540 mn (Gilford Spectrophotometer, USA) for assays per- formed in LAPTg broth and by macroscopic observation for agar tests. Statistical evaluation The disc diﬀusion method was performed by duplicate and the diameters obtained for each strain are represented in the tables. MIC test was performed by triplicate. Complete inhi- bition of growth in all three tubes or plaques with the same antibiotic concentration was considered as the MIC. RESULTS Disc diffusion method Growth of lactobacilli in Mu¨ller Hinton broth was poor and when any type of growth was detected on the agar, it was ir- regular and the halos were undeﬁned. In LAPTg agar the in- hibition halos were sharply deﬁned (Figure 1) and the diame- ters could be easily measured when the microorganisms were inoculated either on the surface or into the agar. On the other side, the diameters of the halos for lactobacilli inoculated on the surface or into the agar were hardly diﬀerent (data not shown). L gasseri CRL1259 and L johnsonii CRL1294 did not grow when they were included into the MRS agar plates while none of the six tested lactobacilli were able to grow in this media when they were spread on the surface. For those strains that were able to grow in MRS and LAPTg agars, the diameters of the inhibition halos were wider in MRS than in LAPTg agar for most of the antibiotics tested, as shown in Table 1. In order to know whether LAPTg or MRS agar was ap- propriate to be used as a base medium in a standardized method for Lactobacillus, the eﬀect of antibiotics on an NC- CLS selected type strain inoculated in this medium was eval- uated. If the halos for the type strain in LAPTg or MRS agar