Death Mechanisms in Airborne Escherichia coli

Abstract

SUMMARYDifferences in survival of Escherichia coli (strain B) sprayed from distilled water into air and into nitrogen as a function of relative humidity (RH) are reported. Two mechanisms which may contribute to death of airborne bacteria are described. In air one death mechanism occurring at low RH is attributed to the action of oxygen causing damage to flavin-linked enzymes as a result of free radical activity. Free radical suppressors are therefore expected to protect airborne E. coli B. Also, electron transport inhibitors like sodium azide, 2,4-dinitrophenol and potassium cyanide are shown to protect E. coli B against lethal effects of oxygen. An analogy is drawn with effects of oxygen on freeze-dried E. coli B. A second death mechanism of E. coli B in air occurs at higher RH's and is considered to result from the effect of aerosolization on RNA synthesis. The activation of RNAse as a possible protection to bacteria in the post-aerosolization medium is discussed. I N T R O D U C T I O NBecause the effect of aerosolization upon bacteria survival is related in a complex manner to relative humidity (RH) of the atmosphere, earlier reports (Dunklin & Puck, 1948;Davis & Bateman, 1960;Bateman, McCaffrey, O'Connor & Monk, 1961) have suggested that bacterial sensitivity increases at certain critical degrees of dehydration. Later reports (Hess, 1965;Cox, 1966) have shown that other factors, e.g. spray fluid, oxygen content of the carrier gas and the collecting fluid, influence the disruption of vital structures and the unbalancing of metabolism. Kinetic studies indicate that : (i) the loss of viability of bacteria is not a simple process; (ii) first-order reactions do not apply; (iii) typically, death rate is initially high and subsequently diminishes progressively.The possibility that overlapping lethal mechanisms exist, makes it extremely difficult to identify bacterial death by a given mechanism in a given set of conditions. One approach to the problem is the use of inert atmospheres to limit the effect of gaseous factors. Another is comparison between results of analogous studies, such as freeze-drying experiments, to derive support for evidence produced in aerosol tests. This paper reports results of such investigations. METHODS Growth of organismsCultures of Escherichia coli (strain B) were grown in 100 ml. of a chemically defined medium for 16 hr at 37", using a shake flask technique. The medium consists of 21-2

Survival of virulentLegionella pneumophilain aerosols

“…Oxygen is known to be toxic to many bacterial aerosols, but often the effect may only be apparent at low r.h. (Hess, 1965;Cox, 1966;Benbough, 1967Benbough, , 1969Webb, 1967); it is possible that the poorer survival of L. pneumophila in drier air may be due in part to oxygen toxicity. One disadvantage of assessing microbial aerosol stability at only three representative r.h. values is that for many bacteria zones of relative instability exist at certain r.h. values (Anderson, 1966;Benbough, 1967;Cox, 1966Cox, , 1969Cox, , 1971) and similar zones of instability have been found for freeze-dried bacteria (Bateman et al 1961;Dewald et al 1967). The survey of the viability of L. pneumophila 74/81 aerosols held for 15 min showed that this organism survives poorly at 55 % r.h. compared with its stability at r.h. values slightly above and below this (65 and 40 % respectively); this suggests that such zones of instability also exist for legionella.…”

Factors Affecting the Toxicity of Oxygen Towards Airborne Coliform Bacteria

“…However, comparison of the oxygen effects on freeze-dried bacteria (Lion & Bergmann, 1961;Lion & Avi-Dor, 1963) with those on airborne bacteria and simulation of the aerosolized state by drying bacterial suspensions on filter membranes (Benbough, 1967) have been successful in attempts to elucidate the death mechanisms of airborne bacteria. In dried bacteria enzymes and/or coenzymes concerned with pyruvate degradation and with the reduction of tetrazolium salts were affected.…”

“…Escherichia coli (strains B, JEPP and COMMUNE) and Klebsiella aerogenes (H. 8) were either grown in a chemically defined medium (Benbough, 1967) or in a complex tryptone medium (Anderson, 1966).…”

“…Cox (1966) and Cox & Baldwin (1967) showed that Escherichia coli sprayed from distilled water died rapidly in air or in oxygen and that death was very marked at low relative humidity (RH) values. At RH values above 70% other death mechanisms prevail (Cox & Baldwin, 1966;Benbough, 1967) which may be associated with rehydration on collecting the partially dried bacteria.…”

The Sensitivity of Gram-negative Bacteria, Recovered from Aerosols, to Lysozyme and Other Hydrolytic Enzymes

“…Bacterial survival in aerosols depends on a number of factors including relative humidity, composition of the gaseous environment and the compositions of both spray and collecting fluids (Webb, 1960;Cox, 1966a, b ;Benbough, 1967;Cox & Baldwin, 1967). Comparatively little is known about the effect of aerosolization upon the integrity of bacterial walls and membranes.…”