Quantitative ecology of psychrophilic microorganisms. Appl. Microbiol. 14:74-. 78.
1966.-To obtain information on the importance of psychrophiles in nature, 95.
APPLIED MICROBIOLOGY, Jan., 1966
Vol. 14, No. 1
Printed in U.S.A.
Copyright © 1966 American Society for Microbiology
Quantitative Ecology of Psychrophilic Microorganisms J. L STOKES AND MARY L. REDMOND Department of Bacteriology and Public Health, Washington State University, Pullman, Washington
Received for publication 9 August 1965
STOKES, J. L. (Washington State University, Pullman), AND MARY L. REDMOND. Quantitative ecology of psychrophilic microorganisms. Appl. Microbiol. 14:7478. 1966.-To obtain information on the importance of psychrophiles in nature, 95 samples of soil, water, mud, and various foods were quantitatively assayed for their content of psychrophilic bacteria and fungi and also for mesophilic and thermophilic bacteria and fungi. Thousands to millions of psychrophilic bacteria were present per gram of soil and represented 0.5 to 86% of the bacterial population. Also, about 25 % of the fungi in uncultivated soil were psychrophilic. In stream and river water, psychrophilic bacteria constituted 16 to 47% of the bacterial population; in lake water, 41 to 76%; and in lake mud, 11 to 33%. Large numbers of psychrophilic bacteria were present in dairy products, meats, and other foods, and accounted for 35 to 93% of the bacterial population of meats. In contrast, thermophilic bacteria usually comprised 1% or less of the bacterial population in all of the materials examined. The data indicate that psychrophiles are both ubiquitous and numerous in nature, and probably play important roles in the cycles of matter.
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ABSTRACT
centrations of psychrophilic bacteria, millions or even billions per gram or square centimeter, occur on or in poultry, fish, meats, vegetables, and dairy products, and also that hundreds or thousands of psychrophilic molds are present per gram of frozen pastries (11, 12). The present investigation was undertaken to determine the quantitative distribution of psychrophilic bacteria and fungi in 95 samples of soil, water, mud, and various foods as determined by counts at 0 C. Simultaneously, quantitative counts were made of the mesophilic and thermophilic bacteria and fungi present for comparative purposes.
A large amount of information on the distribution of psychrophilic microorganisms has accumulated since they were first described by Forster in 1887 (8). The data clearly indicate that psychrophiles are ubiquitous. They occur in temperate as well as polar regions, on land and in water, on and in a large variety of plants and animals, and in foods. The group includes bacteria, yeasts and molds, and also morphological and taxonomic types similar to mesophiles. Thus, psychrophilic bacteria may be long or short rods, cocci or vibrios, gram-positive or gram-negative, sporeformers of nonsporeformers, and aerobic, facultative, or anaerobic. All of them have the common property of being able to grow at 0 C and lower temperatures, in contrast to the minimal growth temperature of about 10 C for mesophiles (7, 10, 15, 17). The significance of psychrophiles in nature is not fully clear, since data on the numbers of psychrophiles in natural habitats are limited. Most of the quantitative data are for foods, especially dairy products (3, 7, 16). Many of these counts are difficult to interpret because incubation temperatures above 0 C were used (sometimes as high as 10 C, which would permit growth of mesophiles). But there is no doubt that high con-
MATERIALS AND METHODS The soils included uncultivated, cultivated, and garden soil. The water samples were from streams, rivers, and lakes, and corresponding samples of lake bottom mud were taken at the same locations. The food samples included milk and other dairy products, chilled meats, frozen vegetables, and precooked frozen foods obtained from local stores. The dry weights of the soil and mud samples were determined by drying portions to constant weight at 105 C. The diluent for the microbial counts was 0.1% peptone. The dairy products were assayed by the procedures in Standard Methods for the Examination of
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VOL. 14, 1966
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ECOLOGY OF PSYCHROPHILIC MICROORGANISMS
although one soil had 15% psychrophiles. Psychrophilic fungi were present, to the extent of 30% in one soil. The uncultivated soils differed markedly from the other two soil types. The total microbial population was smaller, as might perhaps be expected since such soils receive relatively little organic matter. But psychrophilic bacteria were usually predominant and represented as much as 86% of the bacterial population. As usual, relatively few thermophilic bacteria were present, less than 1 %. Considerable numbers of psychrophilic fungi were found, and they accounted for about 25 % of the fungal population. In general, it appears that cultivation favors the growth of mesophilic bacteria, and this reduces RESULTS the percentage of psychrophiles, although hunThermophilic fungi were not found in any of dreds of thousands or millions of psychrophilic the materials examined. Such fungi, which have a bacteria and hundreds or thousands of fungi were maximal growth temperature of 50 to 60 C, present per gram of soil (dry weight)-except for exist, although they are not common (5). It may the absence of psychrophilic fungi in the garden be that our incubation temperature of 55 C was soils. Forster (9) found 140,000 psychrophilic somewhat high, since only a few thermophilic bacteria per gram of garden soil, and Lochhead fungi can grow at 55 C. (13) reported approximately 2,000,000 psySoils. Representative data for psychrophilic, chrophilic bacteria per gram of frozen field soil mesophilic, and thermophilic bacteria and also (compared with 32,000,000 mesophiles). fungi (except thermophiles) are presented in Water and lake mud. The numbers of psychroTable 1. In the garden soils, mesophilic bacteria philic, mesophilic and thermophilic bacteria and predominated, but psychrophilic bacteria averaged fungi in different river and stream waters are about 1 million per gram of soil and accounted shown in Table 2. Hundreds of psychrophilic for as much as 19% of the total bacterial popu- bacteria per milliliter of river water were found, lation. They greatly exceeded the thermophiles, and these accounted for 16 to 29% of the bacwhich represented 0.3 % or less of the bacterial terial population. Mesophiles were predominant population. No psychrophilic fungi were present. (70 to 84%), and thermophiles were relatively The cultivated soils which were from wheat fields scarce (1 % or less). Psychrophilic fungi were had fewer psychrophilic bacteria, as percentage absent. Stream water had a much higher populaof the total population, than did the garden soils, tion of psychrophilic bacteria, usually thousands TABLE 1. Psychrophilic, mesophilic, and thermophilic bacteria and fungi in soils Bacteria Source
Psychrophiles No.*
Per cent
Fungi
Mesophiles
Thermophiles
No.
Per cent
22,000,000 5,000,000 7,200,000
96 84 81
Psychrophiles
No.
Per cent
No.
10,000
