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Appl Microbiol. 1961 November; 9(6): 567–571.
PMCID: PMC1057789

Relative Humidity and the Killing of Bacteria

The Survival of Damp Serratia marcescens in Air


The viability of washed moist cells of Serratia marcescens after storage has been measured in relation to variations in the prior treatment of the cells and in conditions of storage. The factors considered were: (i) water content during storage; (ii) method of arriving at water content (partial drying in vacuum or freeze-drying and addition of water); (iii) presence or absence of air during storage.

Increasingly rapid decay occurs as the water content at which the cells are stored is diminished from above 90% to 20 or 30% (“critical” water content). It occurs in presence or absence of air and it occurs whether the final water content is approached by removal of water from wet cells or by addition of water to freeze-dried cells.

The rate of decay during storage at 20 to 30% water is somewhat diminished by the presence of air (“protective” effect of air).

As the water content is further reduced to less than 10%, the stability of cells stored in a vacuum approaches that of wet cells. In presence of air the reverse is true: the stability decreases until at less than 1% water, the decay rate is about as great as at the “critical” water content (“toxic” effect of air).

Particularly rapid decay of S. marcescens at the “critical” water content has escaped attention in aerosol studies because accurate control of relative humidity (RH) in this region, RH 94 to 99%, is virtually impossible in such studies. On the other hand, values of decay rates referred to measured water contents are quite unreliable in the 20 to 80% RH zone because the corresponding variation of water content is too small to measure reliably. Thus data of the kind reported in this paper cannot be directly compared to the published results of studies of air-borne bacteria, although they are relevant to the practical question of air-borne infection in humid atmospheres.

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Selected References

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  • FERRY RM, BROWN WF, DAMON EB. Studies of the loss of viability of stored bacterial aerosols. II. Death rates of several non-pathogenic organisms in relation to biological and structural characteristics. J Hyg (Lond) 1958 Mar;56(1):125–150. [PMC free article] [PubMed]
  • KETHLEY TW, FINCHER EL, COWN WB. The effect of sampling method upon the apparent response of airborne bacteria to temperature and relative humidity. J Infect Dis. 1957 Jan-Feb;100(1):97–102. [PubMed]
  • MONK GW, MCCAFFREY PA. Effect of sorbed water on the death rate of washed Serratia marcescens. J Bacteriol. 1957 Jan;73(1):85–88. [PMC free article] [PubMed]
  • ELBERT ML, MCCAFFREY PA, MONK GW, STEVENS CL. Effect of water on the death rate of Serratia marcescens. J Bacteriol. 1956 Sep;72(3):368–372. [PMC free article] [PubMed]
  • MONK GW, MCCAFFREY PA, DAVIS MS. Studies on the mechanism of sorbed water killing of bacteria. J Bacteriol. 1957 May;73(5):661–665. [PMC free article] [PubMed]
  • Naylor HB, Smith PA. Factors Affecting the Viability of Serratia marcescens During Dehydration and Storage. J Bacteriol. 1946 Nov;52(5):565–573. [PMC free article] [PubMed]
  • Wells WF, Zappasodi P. THE EFFECT OF HUMIDITY ON BETA STREPTOCOCCI (GROUP C) ATOMIZED INTO AIR. Science. 1942 Sep 18;96(2490):277–278. [PubMed]

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