Silver nitrate and thymol; two disinfectants effective against Legionella pneumophila

Letters

to the

Editor

395

Sir, Silver

nitrate

and

thymol;

two

Legionella

disinfectants

effective

against

pneumophila

We write to report the results of preliminary trials of the effects, under laboratory conditions, of two disinfectant agents, silver nitrate and thymol (2-isopropyl-Smethylphenol), on Legionella pneumophila. These were undertaken following the discovery of L. pneumophila serogroup I in dental waters at the London Hospital Dental School (Oppenheim et al., 1987). Both the agents tested are in regular use in dental practice and are of low toxicity for man. We prepared aqueous solutions of the disinfectants in sterile glass-distilled water (GDW) and in autoclaved tap water to give final concentrations of O-1 % w/v. Plastic apparatus was used for the experiments with silver (which binds to glass) and glass for the experiments with thymol. All the Legionella cultures were grown on Legionella CYE agar with BCYE growth medium and MWY selective supplement (Oxoid). The tests were done on fourteen separate isolates of Legionella pneumophila serogroup I which had been passaged less than three times. The identity of the organisms was checked by tests for failure to grow aerobically on blood agar at 37°C and by fluorescent antibody tests with specific antisera provided by the Central Public Health Laboratory, London. Suspensions of Legionella were prepared by inoculating a heavily charged loopful of the organism (from a culture plate) into 2 ml of glass distilled water in a bijou bottle, adding sterile glass beads and agitating violently on a ‘Whirlimix’ (Hook & Tucker). Uniform dispersion was confirmed microscopically and the number of cfu ml-’ determined by the Miles and Misra method. Suspensions containing approx. 10’ cfu ml-’ were produced. The results of our experiments showed that both agents exerted an antibacterial effect against Legionella pneumophila, the silver acting at a lower concentration than the thymol in distilled water. Tap water reduced the effectiveness of silver (Table I). Tests of the time taken for the Table

I. Results

of titrations

of the two disinfectant

agents

Silver

against

14 isolates

of L. pneumophila

Thymol -

%

GDW

Tap

0.1 0.01 0.001 0~0001 0~00001

+ = Growth,

- = no growth,

GDW

water

%

+ + +

0.1 0.08 0.06 0.04 0.02 0.01 0.001

= glass-distilled

water.

GDW

Tap

water

+ +

+

396

Letters

to the Editor

disinfectants to act showed that both agents acted rapidly, no growth being obtained after the suspensions of the organisms had been in contact with 0.001% silver nitrate or 0.05% thymol for less than one minute. Further tests showed that this was not due to disinfectant carried over onto the plates. Continuous disinfection of dental waters with conventional biocides is generally impractical due either to the toxicity or unpalatability of such agents. Our results suggest that more detailed work on the value of silver salts and thymol (particularly the former) for the disinfection of dental waters would be justified. Such experiments should include tests on other bacterial contaminants of dental waters and should involve experiments using dental equipment. T. D. Healing* B. A. Oppenheim**

Department of Medical Microbiology, London Hospital Medical College, Turner Street, London El 2AD Present addresses: *Communicable Disease Surveillance Centre, 61, Colindale Avenue, London N W9 5EQ **Bacteriology Laboratory, Withington Hospital, Manchester MZO 8LR Reference

Oppenheim,

B.

pneumophilu infection.

A., Sefton, contamination

Epidemiology

A.

and

M., Gill, 0. N. et al. of dental stations in a dental Infection 99, 159-l 66.

(1987). school

Widespread without apparent

Legionella human

Sir, Closed disposable

suction

systems:

need, waste or hazard

With great interest I read the article by Blenkharn & Oakland (1989) and the letter to the editor by Griffiths et al. (1989) about environmental hazards from a hospital incinerator. The authors expressed concern that the amount of liquid present in hospital waste would reduce the temperature of the incinerator chamber leading to poor combustion and an increase in toxic exhaust. I would like to point to some further waste problems arising with use of closed disposable suction equipment. Firstly, and most important of all, there is no convincing study in the literature showing that disposable suction systems prevent cross infection, and reusable suction equipment poses a great hazard as a reservoir of