The bacteria of the paratyphoid group are closely related to the true typhoid bacillus, but differ from the latter organism in being able to ferment glucose with gas production. They are more highly pathogenic for the lower animals than is the typhoid bacillus, but apparently somewhat less pathogenic for man. Most types of paratyphoid bacilli found in food poisoning produce more or less rapidly a considerable amount of alkali, and, if they are inoculated into milk containing a few drops of litmus, the milk after a time becomes a deep blue color. Several distinct varieties of paratyphoid bacilli have been discovered. The main differences shown by these varieties are agglutinative differences. That is, the blood serum of an animal that has been inoculated with a particular culture or strain will agglutinate that strain and also other strains isolated from certain other meat poisoning epidemics, but will not agglutinate certain culturally similar paratyphoid bacteria found in connection with yet other outbreaks. Except in this single matter of agglutination reaction, no constant distinction between these varieties has been demonstrated. The clinical features of the infections produced in man and in the higher animals by the different varieties seem to be very similar if not identical.
The bacillus discovered by Gärtner (loc. cit.) and known as B. enteritidis or Gärtner's bacillus is commonly taken as the type of one of the agglutinative varieties. Bacilli with all the characters of Gärtner's bacillus have been found in meat poisoning epidemics in various places in Belgium and Germany. Mayer[69] has compiled a list of forty-eight food poisoning outbreaks occurring between 1888 and 1911 and attributed to B. enteritidis Gärtner. These outbreaks comprised approximately two thousand cases and twenty deaths. In twenty-three of the forty-eight outbreaks the meat was derived from animals known to be ill at the time, or shortly before, they were slaughtered. Sausage and chopped meat of undetermined origin were responsible for eleven of the remaining twenty-five outbreaks. Two of the B. enteritidis outbreaks were attributed to Vanille Pudding; one, to potato salad.
In other food poisoning outbreaks a bacillus is found which is culturally similar to the Gärtner bacillus, but refuses to agglutinate with the Gärtner bacillus serum. Its cultural and agglutination reactions are almost, if not quite, identical with those of the bacilli found in human cases of paratyphoid fever which have no known connection with food poisoning. Mayer[70] gives a list of seventy-seven outbreaks of food poisoning (1893-1911) in which organisms variously designated as "B. paratyphosus B" or as "B. suipestifer" were held to be responsible. The total number of cases (two thousand) and deaths (twenty) is about the same as ascribed to B. enteritidis. According to Mayer's tabulation meat from animals definitely known to be ailing is less commonly implicated in this type (ten in seventy-seven) than in B. enteritidis outbreaks (twenty-three in forty-eight). Sausage and chopped meat of unknown origin, however, were connected with eighteen outbreaks.
The bacillus named B. suipestifer was formerly believed to be the cause of hog cholera, but it is now thought to be merely a secondary invader in this disease; it is identical with the bacillus called B. paratyphosus B in its cultural and to a large extent in its agglutinative behavior, but is regarded by some investigators as separable from the latter on the basis of particularly delicate discriminatory tests. Bainbridge, Savage, and other English investigators consider indeed that the true food poisoning cases should be ascribed to B. suipestifer and would restrict the term B. paratyphosus to those bacteria causing "an illness clinically indistinguishable from typhoid fever." German investigators, on the other hand, regard B. suipestifer and B. paratyphosus B as identical. My own investigations[71] indicate that there is a real distinction between these two types.
Bearing directly on this question is the discussion concerning the distribution of the food poisoning bacilli in nature. Most investigators in Germany, where the majority of food poisoning outbreaks have occurred, or at least have been bacteriologically studied, are of the opinion that B. suipestifer (the same in their opinion as B. paratyphosus B) is much more widely distributed than B. enteritidis and that it occurs, especially in certain regions, as in the southern part of the German Empire, quite commonly in the intestinal tract of healthy human beings. Such paratyphoid-carriers, it is supposed, may contaminate food through handling or preparation just as typhoid-carriers are known to do. A number of outbreaks in which contamination of food during preparation is thought to have occurred have been reported by Jacobitz and Kayser[72] (vermicelli), Reinhold[73] (fish), and others. Reinhold notes that in one outbreak several persons who had nursed those who were ill became ill themselves, indicating possible contact infection. In another outbreak also reported by Reinhold it was observed that those who partook of the infected food, in this case dried codfish, on the first day were not so severely affected as those who ate what was left over on the second day. A bacillus belonging to the paratyphoid group was isolated from the stools of patients, but not from the dried codfish. These facts were interpreted as signifying that the fish had become infected in the process of preparation and that the bacilli multiplied in the food while it was standing.
There seems no doubt that certain cases of paratyphoid food poisoning are caused by contamination of the food during preparation and are, sometimes at least, due to infection by human carriers. The bacilli in such cases are usually (according to many German investigators) or always (according to most English bacteriologists) of the B. suipestifer type. Other cases are due to pathogenic bacteria derived from diseased animals, and these bacteria are often, possibly always, of a slightly different character (B. enteritidis Gärtner). It is still unsettled whether both types of food poisoning bacteria are always associated with disease processes of man or animals, or whether they are organisms of wide distribution which may at times acquire pathogenic properties. In certain regions, as in North Germany and England, such bacteria are rarely, if ever, found except in connection with definite cases of disease. In parts of Southwest Germany, on the other hand, they are said to occur with extraordinary frequency in the intestines of healthy men and animals. Savage[74] believes that there is some confusion on this subject owing to the existence of saprophytic bacteria which he calls "Paragaertner" forms and which bear a close resemblance to the "true" Gärtner bacilli. They can be distinguished from the latter only by an extended series of tests. The bacilli of this group show remarkable variability, and in the opinion of some investigators "mutations" sometimes occur which lead to the transformation of one type into another.[75]
In spite of the present uncertainty regarding the relationship and significance of the varieties observed, a few facts emerge plainly from the confusion: (1) The majority of meat poisoning outbreaks that have been bacterially studied in recent years have been traceable to one or another member of this group and not to "ptomain poisoning." (2) Bacteria of the paratyphoid enteritidis group that are culturally alike but agglutinatively dissimilar can, when taken in with the food, give rise to identical clinical symptoms in man. (3) Food poisoning bacteria of this group, when derived directly from diseased animals, seem more likely to be of the Gärtner type (B. enteritidis) than of the B. suipestifer type.
Toxin production.—The problem of the production of toxin by the bacteria of this group and the possible relation of the toxin to food poisoning has been much discussed. Broth cultures in which the living bacilli have been destroyed by heat or from which they have been removed by filtration contain a soluble poison. When this germ-free broth is injected into mice, guinea-pigs, or rabbits, the animals die from the effects. Practically nothing is known about the nature of the poisonous substances concerned, except that they are heat-resistant. They are probably not to be classed with the so-called true toxins generated by the diphtheria and tetanus bacilli, since there is no evidence that they give rise to antibodies when injected into susceptible animals. In the opinion of some investigators the formation of these toxic bodies by the paratyphoid-enteritidis bacilli in meat and other protein foodstuffs is responsible for certain outbreaks and also for some of the phenomena of food poisoning, the rapid development of symptoms being regarded as due to the ingested poisons, whereas the later manifestations are considered those of a true infection. Opposed to this view is the fact that well-cooked food has proved distinctly less liable to cause food poisoning than raw or imperfectly cooked food.
A large proportion of the recorded meat poisoning outbreaks are significantly due to sausages made from raw meat and to meat pies, puddings, and jellies. This is most likely because the heat used in cooking such foods is insufficient to produce germicidal results. In milk-borne epidemics also it is noteworthy that the users of raw milk are the ones affected. For example, respecting an extensive B. enteritidis outbreak in and about Newcastle, England, it is stated:
In no instance was a person who had used only boiled milk known to have been affected. Thus in one family, consisting of husband, wife, and wife's mother, the two women drank a small quantity of raw milk from the farm, at the most a tumblerful, and both were taken ill about twelve hours later. The husband, on the other hand, habitually drank a pint a day, but always boiled. He followed his usual custom on this occasion, and was unaffected.[76]