**2. Results**

#### *2.1. Legionella Contamination Rates*

During the study period, 2830 water specimens were collected routinely from the water systems of 168 hotels and resorts in six districts across Israel. *Legionella* spp. were isolated from 470 samples (17%) originating from 102 (60%) hotels. The percentage of *Legionella*-positive samples was lower in the Southern, Jerusalem, and Tel Aviv districts (13%, 15%, and 14%, respectively), of which the largest number of samples was submitted (1139, 794, and 447 samples). A higher level of contamination was found in the North (40%), but only 42 samples were collected (Table 1 and Figure 1). In 230 samples (49% of all positive, 8% of total samples), accounting for 37% of hotels, *Legionella* spp. concentrations exceeded the national regulatory thresholds. The percentage of exceeding samples per district ranged from 6% to 33% (Table 1).

Analysis of *Legionella* spp. prevalence according to sample source showed that both cold and hot water distribution systems were colonized. The leading contaminated water sources were cooling towers (38%), followed by hot tubs, outlets, and main water lines (14–17% each). Of 277 *Legionella*-positive water samples from the outlets representing hotel rooms, 166 (59.9%) and 111 (40.1%) were from hot and cold water systems, respectively. The respective positivity rates were 15.3% and 17.1%. Levels exceeding regulatory *Legionella* counts were detected in 84 samples from the outlets, of which 67 samples (79.7%) originated from hot and 17 (20.2%) from cold water systems. The respective exceedance rates above regulatory thresholds were 6.2% and 2.6%. The majority of contaminated main water lines represented hot water distribution systems and accounted for 63 (92.6%) of 68 positive samples from water lines. Only five (7.3%) positive samples were from cold water lines. Of 35 samples with levels of contamination exceeding regulatory thresholds, 33 (94.2%) were from hot water lines at a rate of 8.6%. In 74 (32%) of 232 samples from cooling towers, and 36 (17%) of 218 samples from hot tubs, levels of *Legionella* spp. exceeded the regulatory thresholds of 1000 CFU/L established for potable water and the 1 CFU/100 mL threshold for the hot tubs (Table 2).

**Figure 1.** Geographic distribution and number of samples obtained from the 168 hotels and resorts included in the study. Samples are represented as pie charts at different locations; the size is proportional to the number of samples obtained from a specific location. Negative samples are shown in green, and positive and exceeding samples are shown in blue and red, respectively. Four major tourist sites with the largest number of samples (negative/positive/exceeding the regulatory thresholds) are Eilat (695/38/34), Jerusalem (662/71/52), the Tel-Aviv region (384/31/32) and the Dead Sea region (210/28/32).


**Table 1.** Distribution of premises and samples, according to administrative region.

1 Cut-off values of 1000 CFU/L for all categories and 1 CFU/100 mL for hot tubs, according to the national regulations; a Out of 168 hotels and resorts tested for *Legionella* spp., 119 were tested more than once (range of 1–25 sampling days, median 2); b Of 101 hotels with positive samples under the regulatory cutoffs, 56 were sampled at least twice (range of 1–17 sampling days, median 2); c Of 62 hotels with *Legionella* concentrations above the national regulatory cutoffs, 29 were sampled at least twice (range of 1–12 sampling days, median 1).

**Table 2.** Prevalence of *L. pneumophila*, according to source type.


1 Cut off values of 1000 CFU/L for all categories and 1 CFU/100 mL for hot tubs, according to the national regulations; 2 According to the national regulations, the water temperature range of 37–39 ◦C for hot tubs can be achieved by mixing hot and cold water. 3 Source type not indicated in the laboratory requisition form accompanying the samples.

#### *2.2. Distribution of Serotypes*

Serotyping of a convenience sample comprised of 162 isolates revealed that 104 isolates (64%) from 44 hotels belonged to sg2–14, and 53 (33%) isolates from 22 hotels were sg1. Both sg1 and sg2–14 were found in the water systems of nine hotels. Two percent of isolates (4/162) for which serogroup identification failed and one recognized by the kit as *Legionella* spp. were subjected to *mip* sequencing, and subsequently identified as *L*. *pneumophila* and *L. bozemanii*, respectively. Overall, Lp accounted for the vast majority of the 162 serotyped isolates (99%).

Furthermore, we analyzed serogroup prevalence according to sample type. The majority of sg2–14 isolates were recovered from hot and mixed water samples, and accounted for 81 (77.8%) of the 104 isolates. Lp sg1 isolates were predominant in cold water systems, and 40 (75.4%) of the 53 isolates originated from cold water.
