Drinking-water quality and issues associated with water vending machines in the city of Los Angeles.

Introduction

Many people concerned about the quality or contents of tap water buy bottled water, filtered tap water, or water from a water vending machine. Waterborne-disease outbreaks, such as the Cryptosporidium outbreak in Milwaukee, Wisconsin, have fueled public concern about tap water (FDA, 2001; McSwane, Oleckno, & Eils, 1994). Retail stores may sell filtered tap water or have a water vending machine inside or outside the store. The quality of water from water vending machines has been questioned, however. A University of Arizona study completed in 1996 found bacteria in one out of four machines tested, though none of the bacteria that were found cause dangerous infections (Rutz, 1996). More recently, the Los Angeles County Agricultural Commissioner/ Weights and Measures Department (LACAC) conducted the 1997-98 Water Vending Machine Pilot Study, testing 279 samples throughout the county. LACAC reported that 26 percent of the water from water vending machines had a heterotrophic plate count (HPC) of greater than 500 colony forming units per milliliter (CFUs/mL), averaging 1,306 CFUs/mL, with five sites positive for total coliform bacteria and one site positive for fecal coliform bacteria (Fiksdal & Shindy, 1998).

The California Department of Health Services (CDHS) Food and Drug Branch, with industry cooperation, subsequently conducted a study of 794 water vending machines in 37 counties (Glacier Water Services, Inc., 2001; Waddell, 1999). CDHS reported that 20 percent of the samples had an HPC of greater than 500 CFUs/mL (637 CFUs/mL was the average, excluding tour with over 10,000 CFUs/mL); 11 samples were positive for total coliforms and none for fecal coliforms (Lee, Terrazas, & Delacruz, 1999). CDHS concluded that additional regulatory, requirements for vended water were unnecessary, but that identified quality control areas deserved more attention. The identified areas were as follows:

* microbial growth in the tubing between the ultraviolet light and the dispensing spout,

* a need for better maintenance of the vending machines to ensure that sufficient minerals are removed from the source water, and

* a need for better maintenance of all required information labeling and the self-closing dispensing doors.

CDHS does not routinely inspect water vending machines, but requires licensing and third-party testing. It also will respond to a public complaint about a particular machine or store, according to CDHS representatives.

Both the LACAC study and the CDHS study detected microbial activity in vending machine water, which may indicate biofilm development. The organisms in the complex matrix of biofilm exude a protective coating, a slime that protects the biofilm from antibiotics, disinfectants, and toxins. Some heterotrophic bacteria form biofilms on any number of surfaces, including the human body. For example, Pseudomonas aeruginosa has been found as biofilm in the lungs of cystic fibrosis patients (Costerton, Stewart, & Greenberg, 1999). One study found an association between Pseudomonas spp. in dental water lines and the same species in the nasal flora of the dental patients (Penn, Sanders, & Sanders 1981). Another study found an association between Pseudomonas aeruginosa in wounds of dental patients suffering front cystic fibrosis and Pseudomonas aeruginosa in biofilms inside dental water lines (Martin, 1987). Pseudomonas aeruginosa, an opportunistic pathogen often associated with waterborne-disease transmission, has a relatively high resistance to disinfection (McFeters, 1990). Dental unit water lines and water vending machines do not have much in common other than that they both provide water; however, the studies cited above demonstrate the ease with which Pseudomonas spp. is transferred from dental unit water to the patient, and the susceptibility of an immunocompromised person to Pseudomonas aeruginosa, even with limited contact.

The study reported here sought to ascertain if vending machine water currently contains high numbers of bacteria and to further investigate the types of microbial growth present.

Water Vending Machines

In California, vending-machine water must meet the same federal quality standard requirements as bottled water, and both must meet all maximum contaminant level requirements set for public drinking water as determined by CDHS (California Health and Safety Code, 2001). Federal and state laws limit coliform numbers in bottled water to 2.2 (most probable number [MPN]) per 100 mL and require any samples positive for total coliforms to be tested for fecal coliforms (Lee et al., 1999). There is no separate quantitative standard for fecal coliforms, although the state must be notified if fecal coliforms are present. The legal limit on total coliforms for public drinking water systems is less than 5 percent positive samples taken from one water supply (Los Angeles Department of Water and Power [LADWP], 2000). There are currently no federal or state regulatory limits for HPC in vended, bottled, or public drinking water (California Health and Safety Code, 2001; Code of Federal Regulations, 2000; Lee et al., 1999). A review article determined that regulation and compliance for bottled and vended water were comparable to that for tap water (Allen & Darby, 1994). Numerous types of bacteria have been found in bottled water according to another review article (Warburton, 1993). One study found an association between gastrointestinal illness and domestic point-of-use water filtration systems (Payment, Franco, Richardson, & Seimiatyck, 1991). No known cases of illness have been traced to consumption of water from a water vending machine.

Water vending machines must be constructed of approved, corrosive-resistant nonabsorbent material that allows easy cleaning and maintenance, with a dispensing spout protected from contamination by a self-closing tight-fitting door or enclosure. Each machine must be designed so that all approved treatment of the water is effective, and so that the water is disinfected prior to delivery to the consumer, either by ultraviolet light or by another CDHS-approved method. The name, address, and phone number of the machine operator must be displayed on each machine in a position clearly visible to the customer. Each water vending machine must have an annual permit issued by CDHS.

California law stipulates that each machine operator must submit a sample of water for testing to a certified laboratory annually for chemical contaminants and biannually for coliforms, and that the results must be submitted to CDHS (California Health and Safety Code, 2001). A local ordinance requires that each water vending machine be inspected by the Plumbing Division of the Department of Building and Safety for potential cross-connections before being placed in operation and that the inspection be indicated by a city permit on the machine. According to a representative of the Los Angeles Department of Building and Safety, the city would check a machine further if a public complaint was made.

The three kinds of machines most commonly located and sampled in this study were manufactured by Glacier Water[TM], AquaStar[R], and Aqua Products, Inc. Other manufacturers and operators included Water Center LLC, Everclear, and Pure Plus Water Systems. Water vending machines treat municipal water by a series of filtration steps that vary in number and combination, depending on the manufacturer. Treatment steps include

* a carbon filter to remove chlorine and other compounds affecting taste and odor;

* a micron or membrane filter to remove tiny particulates;

* a reverse-osmosis system to remove any remaining impurities;

* an ultraviolet sterilization lamp for disinfection; and

* additional carbon, micron, or membrane filters.

Some machines have a built-in sensory system that prevents water distribution if certain processes in the machine cease to function. Maintenance procedures vary. According to one company, the machines are serviced weekly by company technicians, who clean the outside of each machine, check the filters, change the filters as necessary, and test the water for hardness and total dissolved solids (required only for purified water). According to another company, the individual who purchased the machine services the machine, and the company provides technical support and training. With a third company, purchasers may service the machine themselves or may contract with the company for routine maintenance. A fourth company advertises that its own trained technicians service the machines.

Some machines found in this study lacked proper identification. One manufacturer identified by the name stamped on the make and model plate is no longer in business. The majority of the machines tested had a posted phone number. In several cases, the operator was not accessible. The authors either reached a wrong number when the posted phone number was called or, and in two cases, left messages on an answering machine that were not returned.

Study Design

The study comprised analysis of 40 water samples taken from individual water vending machines at identified locations within the city of Los Angeles from January to March, 2001. The Los Angeles Department of Water and Power (LADWP) supplies potable water throughout the selected study area. Eight samples were collected within the five districts in the LADWP service area (Figure 1). Each location was assigned a site code; permit and contact information was collected from each machine.

[FIGURE 1 OMITTED]

Drinking water from each machine sampled was tested in the field for turbidity, temperature, and the presence of chlorine. Samples were analyzed in the laboratory for fungi, heterotrophic bacteria, total coliforms, fecal coliforms and Pseudomonas spp. Twenty to 30 percent of the 40 sampled water vending machines were revisited from March to June, 2001, for a second or a third sample collection and bacterial testing only. The data were analyzed with SPSS for Windows Student Version 10.0, released January 5, 2000. Total coliform and chlorine were given rankings of 1 for present and 0 for absent. For statistical purposes, machine operators were separated into groups and ranked on the basis of accessibility. Machines were ranked according to visual evidence of graffiti, destruction of outer surfaces or attachments, the condition of doors and dispensing spouts, and whether permits were posted (Table 1).

Methods

Field Sampling

Sterilized 250-milliliter (250-mL) wide-mouth polyethylene bottles containing 10 percent sodium thiosulfate were used to collect the first stream of water from the water vending machines. Two samples were collected with 500-mL bottles because of lack of supplies. A second sample was collected to test immediately for turbidity (with a Hach Portalab[TM] turbidometer, Model 16800) and for residual chlorine (with a Taylor DPD test kit). Temperature was taken with a digital probe thermometer. The sample bottles were stored in an ice chest with blue ice and analyzed within 10 hours or less. The authors collected all samples. All samples were processed for bacteriological analysis immediately upon return to the laboratory and were stored in a refrigeration unit in the locked laboratory.

Bacteriological Analysis

Heterotrophic bacteria were enumerated two ways:

1. in poured standard plate count agar (PCA) (DIFCO) with a sample volume of 1 mL, incubated at 33[degrees]C for 48 hours, and

2. in R2A agar (DIFCO) incubated at room temperature (20[degrees]C [+ or -] 3[degrees]C) for 72 hours ([+ or -] 3 hours) after a sample volume of 0.1 mL was spread-plated.

Presence or absence of coliforms was tested with 100-mL sample portions incubated in triple-strength Presence/Absence Broth (DIFCO) at 35[degrees]C for 48 hours ([+ or -] 3 hours). Positive samples were confirmed in Brilliant Green Lactose Bile Broth (Becton Dickenson) incubated at 35[degrees]C for 24 to 48 hours ([+ or -] 3 hours). Positive samples were checked for fecal coliforms by filtering of a 10-mL refrigerated water sample and incubation on M-FC agar (DIFCO) at 44.5[degrees]C [+ or -] 0.2[degrees]C for 24 hours ([+ or -] 2 hours). Fungi were enumerated by spread-plating of 0.1-mL sample portions onto Malt Extract Agar (DIFCO) and incubation at room temperature (20[degrees]C [+ or -] 3[degrees]C) for five days.

The presence of Pseudomonas was tested by streaking of isolates from R2A and PCA plates (three to six colonies from each plate) onto sterile cetrimide agar plates (DIFCO) (five streaks per plate). To confirm positive samples, the authors inoculated 10-mL asparagine broth tubes (DIFCO) with 1.0-mL refrigerated sample portions. Isolates also were tested for presumptive Pseudomonas aeruginosa: 1.0-mL sample portions were inoculated into acetamide broth (DIFCO) and incubated at 35[degrees]C for 36 hours. A color change to purple in the acetamide broth indicated a positive presumptive test for Pseudomonas aeruginosa.

All tests were done in duplicate except where indicated, and sterility controls were performed on each occasion. HPC (PCA) counts from the first sample set were suspect because of possible contamination from the automatic pipet used solely for the HPC (PCA) test. The refrigerated samples were replated on PCA. Temperatures in the refrigerator ranged from 3[degrees]C to 6[degrees]C.

Results and Discussion

Of the 40 machines sampled, over 17 percent either did not have posted operator information or posted operators who could not be contacted. Over 27 percent of the machines did not have the last date of service posted on the machine. Over 27 percent of the machines did not have a door or had a door that was not self-closing. Over 12 percent of the machines had been vandalized. Over 30 percent lacked CDHS permits, 5 percent had a 1999 CDHS permit, and none of the machines had a 2001 CDHS permit.

The test results revealed that 28 percent had a turbidity level of >1 NTUs, and 5 percent had >5 NTUs. These levels exceed the maximum allowed for drinking water that uses surface waters. (Although LADWP is required to test for turbidity, this test is not required for water vending machines.) The machines in which Pseudomonas aeruginosa was detected had turbidity values ranging from 0.4 to 6.0 NTUs. The 6.0-NTU water sample also tested positive for coliforms, although an association between turbidity and coliforms was not demonstrated statistically. Chlorine was detected in 10 percent of the water samples.

Roughly equal numbers of HPC plates from the original sampling had plate counts [greater than or equal to] 500 CFUs/mL or [less than or equal to] 100 CFUs/mL (48 percent and 42 percent respectively, with an average for HPC [PCA] of 889 CFUs/mL, excluding five counts greater than 30,000, and an average for HPC [R2A] of 411 CFUs/mL). Only eight plates had bacterial counts between these two values. LADWP found average HPC (R2A) results ranging from none to over 3,000 CFUs/mL as tested at selected distribution points (LADWP memorandum in MS Excel, May 2001). Sixty percent of the samples had evidence of mold, indicating fungal growth in the machine, most likely in the lines leading to the dispensing spout (Table 2).

Over 30 percent of the water samples tested positive for coliforms; of these, over 20 percent (three of 13) contained fecal coliform bacteria. The percentage of samples with total coliforms was considerably higher than the 2 percent to 3 percent positive reported in a study of bottled-water samples (Warburton, Dodds, Burke, & Johnston, 1992). This result suggests that the vending machines were not as well maintained as the production equipment of a bottled water facility. LADWP reported 0.0 to 0.3 percent of samples positive for coliforms and a fecal coliform range of "not detected" to four per 100 mL in the source water (LADWP, 2000). Tests for Pseudomonas showed 7.5 percent of the samples contained Pseudomonas spp. and 10 percent of the samples contained Pseudomonas aeruginosa (three of the P. aeruginosa positive samples were delayed positives, which were included in the results and statistical analysis).

Twelve sites out of the original 40 were selected for a second round of sampling, and 13 sites out of the original 40 were selected for a third round. Comparison of the original samples to the selected resamples indicated an equal chance that there would be more microbial activity after the machine was serviced, under the assumption that all the machines resampled had been serviced since previous sampling. Two machines had been removed after the initial sampling and could not be resampled. In one case, the machine was not replaced. In the second case, the new machine was not operational and could not be sampled.

The value of the correlation factor does not in and of itself indicate statistical "significance," but it does provide an idea of the strength of association between the two factors. A higher correlation (closer to 1.00) indicates a stronger likelihood that when one factor occurs, the other factor will also occur. No associations were found between the study results and the LADWP districts or the sampling dates. Statistically significant associations were found between operator accessibility and poor machine conditions, permits, and the servicing interval (Table 3). The results indicated that poor machine conditions (i.e., missing doors or missing dispensing spouts), tended to increase as the operator became less accessible to the public. The machines serviced by unknown or inaccessible operators also tended to lack posted date(s) of service or permits. Poor machine conditions increased as the servicing interval increased or was unknown, and as permit information decreased or was unknown. Pseudomonas spp. and Pseudomonas aeruginosa concentrations showed significant correlations with operator accessibility and poor machine conditions (Table 4).

Water temperatures ranged from 9[degrees]C to 25[degrees]C (48[degrees]F to 77[degrees]F), with a mean value around 14.5[degrees]C (about 58[degrees]F). HPC (R2A) counts tended to decrease as the water temperature increased. This result indicates the possibility that as the temperature of the water increased, the number of psychrotrophic or psychrophilic bacteria (biofilm organisms more likely to grow on R2A media) decreased.

The results of this study show that fungi can be found in water from water vending machines. Fungi affect taste and odor. The presence of fungi in the vending machine water is an indication of the quality of maintenance and servicing. Fungi counts tended to increase as the servicing interval increased or was unknown.

The presence of chlorine indicates ineffective removal by activated carbon filters and possible interference with the reverse osmosis process as a result of damage caused by chlorine, which oxidizes and degrades the membrane (Remco, 2002; personal communication by O. Sewer in a graduate class, Department of Health Sciences, California State University-Northridge, 2000). The damage allows particles to remain in the water. An association was observed between the presence of chlorine in the sample water and HPC (PCA) in the original sampling, although no correlation was found with HPC (R2A). HPC (PCA) in the tour chlorine-positive samples ranged from 150 to 8,500, a narrower range than that overall, suggesting that chlorine tended to limit the number of microbes.

The presence or absence of permits was observed to have an association with HPC (R2A) and Pseudomonas aeruginosa, which points to the quality of machine maintenance; an association also was observed between permits and machine condition. A missing permit does not conclusively indicate that the machine is not maintained or regulated. It is possible that the operator neglected to display the permit, although it seems likely that an operator who obtains and maintains a permit would be motivated to display it. In June 2001, however, it also was observed that none of the machines displayed a 2001 state permit. It is the responsibility of the owner or operator of the machine to put the CDHS permit on the machine.

Sampling in this study depended on locating a machine as a consumer would: by seeing one while passing by, or finding one at a nearby store. This approach located some water vending machines with no posted permit and for which the manufacturer and operator were not traceable. That some of these "mystery" machines exist warrants a closer look. The CDHS study pointed to a need by industry to provide complete identification on the machines, and to maintain the door protecting the dispensing enclosure. This study showed that the concern continued to be an issue two years later.

A 1996 assessment of health risks from naturally occurring microbes in drinking water determined that a human health risk was not established on the basis of available knowledge (Edberg, 1996). The consensus of the recent NSF International/WHO Symposium on HPC Bacteria in Drinking Water was that generally there is no significant health risk from HPCs alone in potable water, "absent additional information" (NSF/WHO, 2002). Published risk assessments of HPC and Pseudomonas risk in potable drinking water showed a higher risk only for people taking antibiotics, with risks generally less than one per 10,000 for a single exposure (Rusin, Rose, Haas, & Gerba, 1997). Another review concluded, "Other than well-defined groups (such as burn victims and cystic fibrosis patients), the general population is refractory to infection with Pseudomonas aeruginosa" (Hardalo & Edberg, 1997).

Conclusion

The results of this study showed that fecal coliform bacteria can be present in water from a water vending machine. The presence of coliforms increases the risk of gastrointestinal illness for the immunocompromised. The results of this study also confirmed that water from water vending machines can have high HPC (over 500 CFUs/mL), which confirms findings by the two previous studies of water vending machines conducted by CDHS and LACAC. A high HPC is not in and of itself a known health risk, as determined by CDHS in its study. Someone with a compromised immune system may, however, react differently to the oral introduction of a high number of nonpathogenic or opportunistic bacteria. More study would be necessary before conclusions could be drawn about the potential health risks to a susceptible population from consumption of water from a water vending machine. The presence of an opportunistic pathogen suggests that further evaluation is needed. This study demonstrated that Pseudomonas aeruginosa can be in water from a water vending machine.

This study also demonstrated that water vending machines can supply water relatively free of microbial contaminants. The key controllable difference between high and low microbial activity in a water vending machine seems to be the quality of machine maintenance. The results show that factors involved in machine maintenance have an association with operator accessibility, and that these factors also have an association with microbial activity.

The problem for consumers is that there is no way to know what is going on microbially in a machine when they approach it to purchase water. The consumer is trusting that the machine is delivering what it claims to deliver: quality pure drinking water free of those things about which the consumer is concerned. Consumers may not be getting the quality drinking water they expect.

TABLE 1
Ranking of Nonparametric Data

Rank   Operator         Permit on Machine        Overall Machine
       Accessibility                             Condition

0                       Both state and local     No vandalism
1      Accessible       Either state and local   Vandalized
2      Not accessible   None                     Vandalized and in
                                                   a security cage

Rank   Operator         Machine Door    Dispensing
       Accessibility                    Spout

0                       On and closed   In place
1      Accessible       On and open     Missing
2      Not accessible   Missing

TABLE 2
Results for Original and Follow-Up Site Sampling Data

Site          Turbidity     Mold              R2A
Code           (NTUs)      CFUs/mL          CFUs/mL
                          (average)        (average)

I-A              0.4              0   20
I-B              0.6             30   55
I-C              0.2          2,000   660 (200)
I-D              0.7          3,000   1,400 (--)
I-E (h)          0.3          4,000   490 (2,000) (3,300)
I-F              0.3              0   760 (300)
I-G (h)          0.4             32   2,100 (300) (600)
I-H              0.2            900   750
2-A              0.2             10   20
2-B (c,e)        0.4              0   0 (--)
2-C (c,d)        1.6              0   5 (1,400)
2-D              1.3              0   50
2-E              0.2              0   0 (110)
2-F              2.2              0   0 (70)
2-G              0.3              0   0
2-H              0.3              0   2,500 (3,500)
3-A              0.1            180   100
3-B              4.1              0   0
3-C              5.6             40   50
3-D              0.2              5   0
3-E              0.2              5   10
3-F (c,d,h)      0.5            500   1,500 (1,000) (--)
3-G              0.3              5   30
3-H (e)          0.2            360   870
4-A              0.2             30   70 (89)
4-B              0.2              0   110
4-C              0.5             20   720 (220)
4-D              0.5              0   180
4-E              0.4            110   360 (1,100)
4-F              0.4             35   15 (300)
4-G (e)          1.4            350   200
4-H              0.3             10   0
5-A              0.3              0   150 (--) (10)
5-B (h)          2.5             30   2,000 (300) (60,000)
5-C              0.3             15   500
5-D              1.7              0   0
5-E (e,g)        2.7              0   2 (1)
5-F (c,d)        6.0            390   710 (300)
5-G              0.5             90   40
5-H              2.1              0   20

Site                     PCA               Coliforms
Code                   CFUs/mL               (Y/N)
                      (average)

I-A           800                              N
I-B           400                              N
I-C           1,800 (6)                      Y (a)
I-D           1,100 (--)                       N
I-E (h)       1,400 (1,700) (3,000)            N
I-F           710,000 (f) (48)               Y (a)
I-G (h)       1,600 (1,00) (170)               N
I-H           50                             Y (a)
2-A           100                            Y (b)
2-B (c,e)     1,100 (--)                     Y (b)
2-C (c,d)     120,000 (f) (600)                N
2-D           120                              N
2-E           1,400 (410)                    Y (b)
2-F           50 (150)                         N
2-G           40                               N
2-H           1,700 (3,500)                    N
3-A           600                              N
3-B           30                               N
3-C           50                               N
3-D           5                                N
3-E           230                              N
3-F (c,d,h)   300 (820) (--)                   N
3-G           50                               N
3-H (e)       1,400                          Y (a)
4-A           35 (930)                         N
4-B           830                          Y (1) (a)
4-C           500 (540)                    Y (1) (a)
4-D           12                             N (1)
4-E           1,700 (240)                  Y (1) (a)
4-F           1,700 (130)                  Y (1) (a)
4-G (e)       8,500                          N (1)
4-H           0                            Y (1) (a)
5-A           39,000 (f) (--) (3)          Y (a)(f)/N
5-B (h)       280,000 (f) (200) (25,000)       N
5-C           1,400                            N
5-D           1,400                            N
5-E (e,g)     150 (0)                          N
5-F (c,d)     430,000 (f) (270)              Y (1)
5-G           580                              N
5-H           3                                N

Note: Two significant figures for all values. Follow-up
sampling data, in parentheses, were collected March to
June, 2001.

(a) Negative for fecal Coliforms.

(b) Positive for fecal Coliforms.

(c) Positive for Pseudomonas spp. and Pseudomonas
aeruginosa (range: 1-4 positive isolates).

(d) Delayed positive for Pseudomonas aeruginosa.

(e) Chlorine detected.

(f) Not included in analyses.

(g) Follow-up sample positive for coliforms and fecal
coliforms.

(h) Follow-up sample positive for Pseudomonas spp. And
Pseudomonas aeruginosa (range: 1-2 positive isolates).

(1) 1 PAB only conducted because of lack of supplies.

(--) No sample taken, machine removed, or machine not
operational.

TABLE 3
Significant Statistical Results for Operator Accessibility and
Machine Conditions, and Descriptive Statistics

                      Operator (mean = 1.23; standard
                            deviation = 0.426)

                      Machine    Dispensing    Machine
                       Door        Spout      Vandalized

Pearson correlation   0.540 **   0.484 **     0.437 **

Mean                  0.550      6.67E-02     0.400
Standard deviation    0.832      0.252        0.764

                   Operator (mean = 1.23; standard
                          deviation = 0.426)

                      Date of Last   Presence of
                        Service        Permits
                       ([dagger])

Pearson correlation    0.789 **      0.480 **
Mean                  11.9           1.30
Standard deviation    12.8           0.646

N = 60.

* Significant at .05 level.

** Significant at .01 level.

([dagger]) Substituting an assumed value for missing
service date information.

TABLE 4
Statistical Results for Pseudomonas aeruginosa and Pseudomonas spp.

                      Pseudomonas aeruginosa (Pseudomonas spp.)

                           Operator             Dispensing
                         Accessibility            Spout

Pearson correlation   0.406 ** (0.372 **)   0.344 ** (0.268 *)
Mean                  1.23                  6.61E-02
Standard deviation    0.426                 0.252

                      Pseudomonas aeruginosa (Pseudomonas spp.)

                         Date of Last      Presence or Absence
                      Service ([dagger])       of Permits

Pearson correlation   0.285 * (0.285 *)    0.287 * (0.218)
Mean                  11.9                 1.30
Standard deviation    12.7                 0.646

N=60.

* Significant at .05 level.

** Significant at .01 level.

([dagger]) Substituting an assumed value
for missing service date information.

Pseudomonas spp.: Mean = 3.095E-02; standard deviation = 0.120.

Pseudomonas aeruginosa: Mean = 0.167; standard deviation = 0.457.

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Corresponding Author: Suzanne Du Vall Knorr, 800 S. Victoria Ave., Ventura, CA 93009-1730. E mail: SuzanneDuvallKnorr@ mail.co.ventura.ca.us.