The revival of formaldehyde-treated bacteria

DISINFECTION of fabrics with gaseous formaldehyde

For a study of the process of formaldehyde disinfection, cotton threads were used as test objects. These were, in most experiments, impregnated with a heavy suspension of a coagulase-negative micrococcus in 1% gelatin solution or in 90% horse serum, and dried. In other experimentsMycobacterium tuberculosis, Bacillus subtilisor smallpox virus was used. After exposure to formaldehyde, quarter-inch lengths of the threads were cut off and cultivated on agar plates; individual threads were recorded as fertile or sterile and the mean number of organisms per thread was estimated statistically. Neutralization of the formaldehyde was found to be unnecessary except when strong concentrations were used.

In laboratory tests there was a linear relation between the concentration of formaldehyde and the killing rate. Little effect was observed on the speed of disinfection from variation in temperature over the range 0–30°C. There was some suggestion of an optimum relative humidity for disinfection at about 80–90%, but no great increase in disinfection rate was obtained by increasing the relative humidity above 58%. Threads impregnated from a suspension of the cocci in 90% serum were disinfected much more slowly than those impregnated with cocci suspended in a solution of gelatin. Covering the threads with several layers of blanket also slowed the killing rate considerably.

Tests were made of the sterilization of the threads in full-size disinfectors. Mattresses and other bedding absorbed a large quantity of formaldehyde so that the amount supplied had to be greatly in excess of that estimated from laboratory tests to provide a lethal concentration. A disinfector working at ambient temperature was found to kill over 99.99% of the micrococci on threads exposed on the surface of mattresses. The destruction of micrococci on threads placed under pillows or blankets was less, as was that of tubercle bacilli on fully exposed threads.

A disinfector in which the air temperature could be raised to about 100°C. was found to be more effective, partly because the increased temperature allowed much greater penetration of fabrics by the formaldehyde vapour; but the relative contribution of the heat and the formaldehyde to the disinfection process could not be fully determined.

In both the laboratory and the field experiments it was found difficult to obtain completely reproducible results. This constitutes one of the most disconcerting features of disinfection by formaldehyde. The process is influenced by so many different factors that it is impossible to predict with real assurance the result under any given set of conditions.

It is concluded that disinfection by formaldehyde vapour should be used only when no other method is available. It certainly cannot be recommended for disinfection of fabric contaminated with smallpox virus or with anthrax spores, when complete destruction has to be ensured; and unless carried out with special care it is not really suitable for woollen garments and toys soiled with tubercle bacilli.

Steam sterilization of air turbine dental handpieces

The efficacy of autoclaving high-speed dental handpieces was tested in two types of downward displacement steam sterilizers (instrument autoclaves), commonly used in the dentist's office. Eight series of experiments were performed with various sterilization schedules. Lubrication oils with or without antimicrobial agent were used in four of the series. Each handpiece was inoculated with 1 ml of a suspension containing equal amounts of saliva and spores of Bacillus stearothermophilus (approx. 10(6) spores/ml). Neither sterilization at 120-124 degrees C for 20 min nor at 134-136 degrees C for 10 min resulted in complete inactivation of the spores in series in which the instruments were tested without oil or with oil containing no antimicrobial agent. However, when the handpieces were lubricated with oil containing isopropanol and formaldehyde and the schedule 134-136 degrees C/10 min was used, no growth was observed. The results indicate that instrument autoclaves with built-in programs of 120-124 degrees C/20 min and 134-136 degrees C/10 min could have insufficient capacity to sterilize lubricated or unlubricated dental handpieces. The use of oils containing an antimicrobial agent can overcome this problem.

The use of inactivators in the evaluation of disinfectants

Since the earliest days of disinfectant testing bacteriostatic effect has misled many workers in this field. This problem has long been appreciated by some and a considerable battery of neutralizers has been employed, often with great success. Other anti-bacteriostatic measures, used without control, fail to revive damaged though viable organisms.This paper sets out to describe some of the problems that are encountered in this confused field and the means whereby they may be overcome.

Effect of sub-lethal treatment with formalin on the germination of Aspergillus fumigatus spores

Sub-lethal exposure of Aspergillus fumigatus spore suspensions to formalin resulted in prolongation by 1-22 days of the period of less than one day normally needed by spores to produce visible growth in Sabouraud's liquid medium at 37 degrees C.; the degree of delay depended on the concentration of formalin and the duration of exposure, and was due to an increase in the germination-time of spores. The formalin concentration could be adjusted so as to affect the germination-time of almost all spores in a suspension without reducing viability. The effect on germination was not abolished by thorough washing or treatment with sodium sulphite. The spores of four different strains of A. fumigatus and of cultures aged 3 to 14 days reacted similarly to formalin treatment. Although of greatly reduced virulence for mice, affected viable spores were still capable of producing infection and death following intravenous inoculation, provided they were not eliminated by the host before germination occurred.