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Parasitologic Surveys in Cali, Departamento del Valle, Colombia. V. Capacity of Entamoeba Histolytica of Human Origin to Utilize Different Types of Starches in its Metabolism

Ernest Carroll Faust^* AND Thomas R. Read

1. Forty % of 1,000 persons in 100 representative families of an underprivileged population group in ward Siloé, Cali, Colombia harbor strains of Entamoeba histolytica, 80% of which belong to the large race; yet clinical manifestations of this infection are rare. The principal food of these persons consists of starches, derived mostly from cooked green plantains and yucca tubers, less commonly macaroni, unfortified bread, Irish potatoes, corn grits, rice and, rarely, white sweet potatoes. Protein and fat supplements, mostly from plant products, are consumed in meager supply and, except for vitamin A in plantains and yucca, vitamins are very inadequate in the diet, while essential minerals are deficient.

2. Because undigested starch grains, identified as those of plantain and at times yucca, are frequently evacuated in the stools of these individuals, it seemed desirable to investigate the capacity of E. histolytica to ingest and utilize the starches consumed by the population to serve the metabolic needs of the parasite.

3. A large race E. histolytica, isolated from human feces for pathogenic studies in guinea pigs, was first cultured in vitro with rice starch in Balamuth's monophasic medium. Following a number of subcultures, starches obtained from plantain, yucca, maize, Irish potato, white sweet potato and wheat were substituted for the rice starch, which served as control in parallel tubes. Evidence of satisfactory utilization of these substituted starches was obtained for plantain, Irish potato and white sweet potato (22 subcultures), yucca (28 subcultures) and wheat (29 subcultures), each continued until the experiments were terminated; in contrast, trophozoites in the maize starch died out after the 16th subculture. In addition, two non-Colombian strains of large-race E. histolytica ("200" and "H") were later tested in New Orleans, employing the same Colombian yucca and Irish potato starches in modified LES medium. Both strains started to grow vigorously with the substituted starches; "200" continued through 16 subcultures but "H" died after a few transfers.

4. The experiments demonstrated the ability of a Colombian strain of E. histolytica to propagate satisfactorily in vitro when plantain, yucca, Irish protato, white sweet potato and wheat starches were substituted for rice starch in the culture medium, partial success with corn starch, and for yucca and Irish potato starches to serve the metabolic needs of one non-Colombian strain ("200") through numerous transfers.

5. In considering the culture tests with respect to the infected population, controlled nutritional studies in many parts of the world indicate that inadequate protein intake such as exists in our population provokes deficient pancreatic and intestinal enzyme production (e.g., amylase and maltase), thus probably explaining the presence of plantain and yucca starch grains in the stools of these persons.

6. Biochemical studies have demonstrated that E. histolytica elaborates amylase and maltase, capable of splitting ingested starch grains and converting them into sugars. The degree of utilization depends on whether the starches contain large amounts of amylopectin (branched-chain component, most easily hydrolyzed by the ameba) or principally amylose (straight-chain component, more resistant to hydrolysis by the ameba). Rice and Irish potato starches consist mostly of the former component, maize, wheat and sweet potato starches of the latter, while the other two starches tested are intermediate in their molecular configuration.

7. Relating the tests carried out in this study with nutritional investigations and the biochemical analyses, it seems permissible to consider the sequence of events as follows: (a) protein malnutrition produces pancreatic and intestinal enzyme deficiency so that ingested starches are inadequately hydrolyzed, with consequent passage of intact starch grains through the intestine and their evacuation in the stools; (b) E. histolytica trophozoites colonizing in the large intestine utilize certain of these starches for their metabolic needs and thus are usually able to maintain their colonies in the intestinal crypts without deep invasion of the intestinal wall; (c) this last link in the chain may explain why the infected population harbors E. histolytica as an essentially asymptomatic infection, even though collateral studies have demonstrated that large race strains of this ameba in this geographic area are highly pathogenic.

8. Although biochemical tests and our studies indicate that size of the starch grains is not the only factor concerned with the ameba's ability to utilize intact starch grains present in its immediate environment, if trophozoites belonging to a small race are physically incapable of ingesting the starch grains, a plausible explanation is provided for the difficulty frequently experienced in getting small races into culture, even when the starch is of a demonstrably utilizable type.

^* Field Coordinator, Tulane-Colombia Program in Medical Education, International Cooperation Administration, Mutual Security Program of the United States Operational Mission to Colombia.

Present address: Fullerton Junior College, Fullerton, California.