Enumeration of Legionella CFU by Colony Hybridization Using Specific DNA Probes

ABSTRACT Oligonucleotide probes and colony hybridization (CH) were applied to enumerate organisms of the genus Legionella in cooling tower water. The CH counts indicated almost the same results as CFU counts in cultivated samples derived from the water. It was concluded that it is possible to substitute the CH procedure for the conventional one.

Legionella is the etiologic agent of Legionnaires' disease. The combination of isolation on selective medium and identification by DNA probe is useful in attempting to detect proliferating Legionella organisms (1,2). Here we report two kinds of oligonucleotide probes, one targeting 16S rRNA and the other targeting a new target region, namely, an antigen common to the genus Legionella. Subsequently, we applied these probes to enumerate viable legionellae in various cooling tower water samples by using colony hybridization (CH). As a result of this investigation, we successfully identified and detected pinpoint Legionella colonies by using each of the probes.
The synthesized oligonucleotides were labeled with a digoxigenin oligonucleotide tailing kit (Boehringer Mannheim, Mannheim, Germany), in accordance with the manual.
Bacterial strains. The bacterial strains used in this study, along with their sources, are listed in Tables 2 and 3. Legionella strains were obtained from the stock strains of the Department of Microbiology, Gifu University School of Medicine. The reference strains were supplied by the Department of Bacteriology, Juntendo University.
Reference microorganisms except Staphylococcus aureus were cultured in Luria-Bertani broth overnight and harvested. About 10 8 to 10 9 cells were suspended in 2 ml of 10 mM Tris-HCl-1 mM EDTA buffer (pH 8.0). One hundred l of 10% SDS (final concentration, 0.5%) and 20 l of 10-mg/ml proteinase K solution (final concentration, 100 g/ml) were added. After the solutions were mixed, the mixture was incubated for 1 h at 37°C. Then 353 l of 5 M NaCl and 282 l of cetyltrimethylammonium bromide-NaCl were added, and the solution was incubated for 10 min at 65°C. Staphylococcus aureus was cultured overnight in Luria-Bertani broth and harvested. The cells were washed twice in 2 ml of 10 mM Tris-HCl-10 mM EDTA buffer (pH 8.0). The pellet was suspended in 2 ml of 10 mM Tris-HCl-10 mM EDTA buffer (pH 8.0) and then mixed sufficiently with 40 l of achromopeptidase (50,000 U/ml in 10 mM NaCl). The solution was incubated at 55°C until the cells were lysed.
The subsequent DNA extraction and purification procedures were performed as described previously (7).
Detection of legionella colonies. The cultivation and identification of legionella colonies from the cooling tower water were performed as described previously (1). The replicated agar plates were reincubated for another 2 to 3 days at 37°C until the colonies grew to a visible size.
Testing of oligonucleotide probe specificity. One hundred ng of nucleic acid was blotted onto a 0.22-m-pore-size Biodyne A nylon membrane (Poll Europe Ltd.). Subsequent dot blot hybridization was performed under the standard 5ϫ SSC pro-    CH. The concentrated cooling tower water was cultured on a WYO␣ agar plate at 37°C for 2 to 5 days. Colonies were transferred from plates to 0.22-m-pore-size (82-mm-diameter) Biodyne A nylon membranes (Poll Europe) by using procedures described previously (7). After replication, the mem-branes were placed colony-side-up on the paper (3MM; Whatman) and were saturated with 5% Triton X-100 in 10 mM Tris-HCl-1 mM EDTA (pH 8.0) for 1 h at 60°C. The membranes were transferred onto the paper (3MM); supplemented with 0.1 mg of proteinase K per ml in 10 mM Tris-HCl (pH 8.0) containing 10 mM NaCl, 1 mM sodium citrate, and 1.5% SDS; and incubated for 1 h at 60°C to lyse the cells. Subsequently, denaturing and neutralization of the DNA were performed as described previously (7). In order to remove the protein, the DNA fixed in the membranes was incubated overnight at 37°C with gentle agitation in a solution including 0.1 mg of proteinase K per ml, 10 mM Tris-HCl (pH 8.0), 150 mM NaCl, and 0.5% SDS. The membranes were washed twice for 5 min at room temperature with about 50 ml of 6ϫ SSC per 100 cm 2 . Hybridization was carried out by using the same procedure described for dot blot hybridization, except that 10% blocking reagent was used. Specificity of the oligonucleotide probes. Probe LEG225 hybridized to 32 of the tested strains of Legionella spp. (91%) ( Table 2). No false-positive hybridization occurred with the reference strain (Table 3). Probes CA/G L253 and CA/G L301 could hybridize 26 strains (74%) and 18 strains (54%) of the Legionella species tested, respectively. A mixed probe consisting of probes CA/G L253 and CA/G L301 together could hybridize 100% of the Legionella species tested ( Table 2), but false-positive hybridization occurred with Klebsiella pneumoniae (Table 3).
Other bacteria and fungi which grew on the WYO␣ agar plates frequently disturbed the growth of legionellae and of the screened legionella colonies. However, as shown in Fig. 1, the signals from Legionella colonies clearly discriminated among these other microorganisms. A Legionella colony formed within a fungus colony was also detected (Fig. 2).
The results of CH counts of organisms from cooling tower water by using probe LEG225 and the results of CFU counts by cultivation revealed a close relationship, with a regression coefficient of 0.96 (Fig. 3).
In the case of the mixed probe consisting of probes CA/G L253 and CA/G L301 together, the results of CH counts were generally in agreement with those of CFU counts by plate culture; the regression coefficient was 0.98 (Fig. 4). The results of CH counts were slightly lower than those yielded by CFU counts of WYO␣ agar plate cultures. This tendency became remarkable in the samples that contained more than 100 CFU per 100 ml. This phenomenon is considered to result from the creation of large spots on the membrane by individual adjacent colonies because the colonies were aggregated after the process of colony transfer and lysing. But this phenomenon did not significantly affect the result of Legionella enumeration. The CH method has excellent features, including the ability to simultaneously perform enumeration of colonies and identification of every colony on a single plate.
Grimont et al. (2) reported that pinpoint Legionella pneumophila colonies were detectable by the use of a DNA probe labeled with a radioactive isotope after 4 days of incubation on BCYE␣ agar. In our study, we demonstrated the ability to enumerate Legionella colonies by DNA probe without radioactive-isotope labeling after the samples were cultured for 3 days.