A Correlation Study of the Colon-Aerogenes Group of Bacteria, with Special Reference to the Organisms Occurring in the Soil

The classification of coliform bacteria

A study of the biological characters of 1636 cultures of coliform bacteria, isolated from milk and bovine faeces, shows that the coliform group consists of a large number of different types. These types are so closely interlinked in characters and in relations to environment as to justify their inclusion in one genus. Nevertheless, to facilitate the identification of types, the group may be subdivided into subgroups, the Voges-Proskauer, Koser, inositol and indole reactions being reliable and outstanding criteria for this purpose. These characters show almost perfect correlations with various other characters. Thus Voges-Proskauer-negative types have a low CO2to H2ratio and are methyl-red-positive; Voges-Proskauer-positive types have a high CO2to H2ratio, are methyl-red-negative and Koser-positive. Koser-negative types are not highly resistant to brilliant green, are non-capsulated, do not form thick mucoid colonies and are Voges-Proskauer-negative, inositol-negative and indole-positive. Koser-positive types are highly resistant to brilliant green, and with the exception of group 2 and certain members of group 3, are frequently encapsulated and form thick, mucoid colonies. Non-inositol-fermenters are as a rule motile and adonitol-negative. Inositol-fermenters are Koser-positive, adonitol-positive, sucrose-positive and raffinose-positive; are frequently encapsulated and form thick, mucoid colonies; and as a rule are non-motile. Indole-negative types are Koser-positive, while indole-positive types are generally Koser-positive or negative according to whether the inositol reactions are positive or negative.

The Significance of Bact. aerogenes in Water

1.Bact. aerogenesis practically universally present, although in small numbers in the stools of normal adult humans. The stools of one individual, however, repeatedly gave negative results. The organism has also been isolated from the faeces of horse, cow, sheep, pig, dog, cat, wild rabbit, wild rat and wild mouse.

2. The use of modifications of the citrate medium, containing lithium or barium, is useful in the isolation ofBact. aerogeneswhenB. pyocyaneusis abundant.

3.Bact. aerogenespredominates overB. coliin soil, the contamination of which by faecal material was unlikely. TheB. coliisolated from such soil do not show differences by the tests used from typicalfaecalstrains ofB. coli.

4. In the municipal water supply of Liverpool the proportion ofBact. aerogenestoB. coliis relatively high and increases on storage.

5. In water contaminated with faeces, the proportion ofBact. aerogenestoB. coliis relatively low but is rapidly reversed on storage. This is largely due to the death of theB. coli, but may in part be also due to multiplication of theBact. aerogenespresent.

6. Preponderance ofBact. aerogenesoverB. coliin a water supply is indicative of either (a) contact with soil which is not contaminated with fresh faeces, or (b) long past faecal contamination.

7. Preponderance ofBact. aerogenesoverB. coliin a water supply may, for practical purposes, be regarded as an indication of freedom on the part of the water from pathogenic organisms, includingB. typhosusandB. paratyphosusB.

8. The repeated examination of faeces during storage showed a marked increase followed by a gradual decrease in the total number of organisms present. The decrease coincided with an increase in the numbers ofBact.aerogenesrelative to the organisms present. Whether an absolute increase in the numbers ofBact. aerogenesoccurred, was not determined.

9. A plea is made for uniformity among the tests adopted by future workers for the identification of coliform organisms, and an exact statement of the methods employed.

The writer desires to thank Prof. J. M. Beattie for helpful advice throughout the course of the investigation.

B. coli as an Index of Faecal Pollution of Water Supplies

The present investigation deals with the routine bacteriological examination of 525 samples of water, 265 of which contained lactose fermenting bacilli; 1441 strains were isolated and various differential tests were used in the classification of these organisms.

All the strains were tested in dextrose phosphate culture for the methyl red and the Voges and Proskauer reactions, and although most of them gave normal results (M.R. + V.P. 0 or M.R. 0 V.P. +), there were 25 strains, 1·73 per cent., which gave either double positive or double negative reactions. Similar cases have already been discussed in an earlier portion of this paper, and they leave no doubt that there are types which may vary from the normal as far as the methyl red and the Voges and Proskauer reactions are concerned. At the same time these abnormal types are not sufficiently numerous to make any real practical difficulties in the use of these tests for routine purposes.

The Koser test, considered by many bacteriologists to be of great differential value, was found to give imperfect correlation with the other reactions.

1395 strains were isolated which could be classified in theB. coligroup as a result of the production of acid and gas in lactose, the clotting of milk, and the non-liquefaction of gelatine, but when the methyl red and the Voges and Proskauer results were considered it was found that nearly 11 per cent. of these strains were of theB. lactis aerogenestype, and were therefore of no value as indicators of faecal pollution.

The indol reactions were somewhat variable. Among 1244 strains of theB. colitype (M.R. + V.P. 0), more than 10 per cent. failed to produce indol; while among 151 strains of theB. lactis aerogenestype (M.R. 0 V.P. +), nearly 51 per cent. were indol producers. Preliminary extraction with ether was carried out with 448 of these strains before Böhme's reagent was added. With this technique it was found that among 400 methyl red positive types there were 87 strains (22 per cent.) which produced no indol, and among 48 methyl red negative strains there were 12 (25 per cent.) which gave the reaction. Chen and Rettger (1920) were unable to establish any definite correlation between the indol test and the other reactions.

When the sample results were studied, it was found that although 262 waters contained organisms which were included in theB. coligroup on the basis of the lactose, milk and gelatine tests, nearly 6 per cent. of these samples contained only organisms which gave a methyl red negative, Voges and Proskauer positive reaction, and were probably associated with pollution from soil washings rather than contamination from faecal sources. This means that the application of the methyl red and Voges and Proskauer reaction does make a considerable practical difference in the interpretation of results in the bacteriological examination of water.

More detailed examination as regards the fermentation of the various carbohydrates is useful for purposes of classification, but is of no significance, in the light of our present knowledge, in assessing the probability of excretal pollution.

It is suggested that, in considering the results of a bacteriological examination of water, organisms should be regarded as significant which are of typical morphology and staining reaction, produce acid and gas in lactose, fail to liquefy gelatine, clot milk, and give a positive methyl red and a negative Voges and Proskauer reaction. The production of indol in a medium containing peptone may be added as an extra test if desired, but the available evidence does not suggest that failure to produce indol is a sufficient basis for the exclusion of an otherwise typical bacillus from theB. coligroup.

My thanks are due to Dr G. D. Dawson, in conjunction with whom the present investigation was started; and also to Prof. W. W. C. Topley whose interest throughout the course of the research was made manifest by his helpful advice and criticism.

The Distribution and Sanitary Significance of B. coli, B. lactis aerogenes and Intermediate Types of Coliform Bacilli in Water, Soil, Faeces, and Ice-Cream

This section of the work deals with the routine bacteriological examination of 2144 samples of water, of which 1102 contained lactose-fermenting bacilli, 4333 strains being isolated. Twelve strains which liquefied gelatine and 24 which gave anomalous MR and VP reactions were excluded from the coliform group; the remaining 4297 cultures were classified on the basis of the MR, VP, indol, citrate and uric acid tests.

The ecology and significance of the different types of coliform bacteria found in water: A review of the literature

1. A review has been made of literature on the ecology of different types of coliform bacteria. The main object of the review has been to consider whether there is evidence to support the view thatBact. aerogenesand the intermediate types live normally on plants or in the soil and not in the intestines of man and other animals.

2. There is ample evidence thatBact. coliis by far the most common type of coliform in normal human faeces. On the other hand, there is evidence thatBact. aerogenesor intermediate types are usually present in faeces, may sometimes be present in greater numbers thanBact. coli, and on rare occasions may be the only type present. Both quantitatively and qualitatively the coliform flora of the faeces of an individual person may vary from day to day. There is insufficient evidence on the numbers ofBact. aerogenesand intermediate types in faeces to justify any more definite statement, but limited data suggest that such types may be absent or may be present in numbers of the order of a million per gram.

3. When fresh faeces are stored there is first a multiplication of such bacteria as will grow on ordinary laboratory media, including the coliform types. The rate of multiplication, as with the flora of soil, water, and milk, increases with an increase in the incubation temperature to 37° C, but the period of multiplication becomes shorter. In the literature consulted no evidence can be found to show which groups are prominent in the multiplication. Results are in agreement that on further storage the ratio of the numbers ofBact. colito those ofBact. aerogenesand intermediates decreases, the typicalBact. coliflora dying off more rapidly than other conform types. The rapidity of decrease appears to depend partially at least upon the temperature of the environment, and the decrease may be accelerated by intense sunlight.

4. In urine from patients suffering from genito-urinary infections the dominant types of coliform are usually eitherBact. aerogenesor intermediates. No data on the number of such organisms in urine have been obtained from the works consulted.

5. There is no-evidence that coliform bacteria multiply on fresh grasses or grains. Few quantitative data on this question have been found. In some of the older work it is doubtful whether a large proportion of the cultures isolated were actually coliform bacteria or whether they were species of other genera capable of fermenting lactose at 30° C. but not at 37° C. In the decomposition of grasses and legumes during ensilage, a process involving a considerable increase in temperature, it would appear that multiplication of conform bacteria may take place and counts may for a time equal those found in fresh faeces. No indication has been found that this multiplication is confined toBact. aerogenesor intermediates.

6. Most workers who have studied the coliform bacteria in soil have ignored the quantitative aspects and no counts at intervals over long periods of coliform bacteria in any undisturbed soil appear to have been made. No evidence of any multiplication of coliform bacteria in soil has been found. Results, however, are in agreement that where pollution of the soil by animal excreta has taken place, the heavier the pollution the greater is the number of coliform bacteria; soils relatively free from human or other animal pollution either contain no coliform bacteria or only small numbers. It is generally agreed that the ratio of the numbers ofBact. colito those ofBact. aerogenesand intermediates decreases with the increase of time which has elapsed since pollution of the soil. This change is similar to that which occurs in faeces during storage.

7. There is insufficient evidence to justify the definite statement often made thatBact. aerogenesand intermediates are normal inhabitants of soils, grasses, and grains.

The Classification of the Colon-Aerogenes Group of Bacteria in Relation to their Habitat and its Application to the Sanitary Examination of Water Supplies in the Tropics and in Temperate Climates: A Comparative Study of 2500 Cultures

(1) The ”lactose+ indol+” index as employed in temperate climate is usually an adequate criterion of water purity.

(2) Where a positive test does not appear to be substantiated by the sanitary findings, a further differentiation of the organisms isolated becomes necessary. The methyl-red and citrate tests have been found to supply that differentiation in a satisfactory manner.

(3) In the Tropics, false positive “lactose-indol” tests are commonly encountered, owing to the presence in large numbers in soil and water of organisms derived from sources other than recently excreted faeces.

(4) In the Tropics, the lactose-indol test should always be confirmed by the methyl-red, citrate and saccharose tests or such other reactions or groups of reactions as may be found by a local survey to be applicable.

My very grateful thanks are due to Dr P. A. Clearkin, Deputy Director of Laboratory Service, Tanganyika Territory, for his ready advice and constant encouragement; and also to Mr W. Whitley, Analytical Chemist, who performed a chemical analysis of the water samples, and very kindly gave me the benefit of his opinion on them. In addition I have to thank Dr R. R. Scott, Medical Officer of Health, and the Staff of the Health Office, Dar-es-Salaam, who have rendered every facility in the choice and collection of samples. It is a pleasure to be able to record once more my indebtedness to Prof. Bigger, of Trinity College, Dublin, whose fund of valuable suggestions and kindly criticism has been ever at my disposal.

68. The Occurrence of Coliform Bacteria in Milk

1. Of 21,569 samples of mixed cow's milk taken under fair conditions of cleanliness and examined by means of the coliform test, 48·3 per cent, gave negative results with 1/10, 1/100 and 1/1000 c.c. amounts; 21–4 per cent, gave positive results with only 1/10 c.c.; 14·0 per cent, gave positive results with only 1/10 and 1/100 c.c.; and 16·3 per cent, gave positive results with 1/10, 1/100 and 1/1000 c.c. It is probable that these results are better than those generally obtained.

2. The proportion of coliform-positive samples was much higher in summer and early autumn than during the winter and spring. This seasonal variation was largely due to atmospheric temperature, there being a well-marked direct correlation between the proportion of coliform-positive samples and the mean of the minimum and maximum atmospheric temperatures.

3. Of 21,857 samples examined, 10,458 were coliform negative and had an average bacterial content of 25,294 per c.c; 11,399 were coliform positive and had an average bacterial content of 160,577 per c.c. The coliform-positive samples contained on an average 6·3 times as many bacteria as the coliform negative.