Basically you are asking if prior colonic infection with Entamoeba coli, a non-invasive amoeba, will protect infectees from later infection with Entamoeba histolytica, an invasive amoeba that causes significant disease. You are also wondering about interactions with other non-bacterial colonic pathogens.
The quick answer is that coinfection is common but I cannot tell you whether there might not be interference. You would need to show that co-infection rates (or polyparasitism) were less common than would be expected on the basis of the prevalence of the individual organisms.
You would also need a sample size that was sufficiently large to have a high expected rate of polyparasitism. What follows is material that may bear on the methods and results of the past. You should also realize that Entamoeba coli is pretty much ignored by physicians, so unless they were specifically looking for it, they would might not record the results.
The first issue to address is whether E. histolytica has been properly identified in the past. See this paragraph from the Entamoeba Homepage: Entamoeba Homepage article
"In many publications Entamoeba histolytica is cited as infecting one tenth of the world population, or 500 million people. In recent years a new understanding of this organism has lead to the recognition that there are in fact two species within what has previously been known as E. histolytica. Of these two organisms, E. histolytica is the cause of all invasive disease while the other, E. dispar, is not capable of invading tissue. These organisms were previously known as ‘pathogenic E. histolytica’ and ‘nonpathogenic E. histolytica,’ respectively, and these names will be encountered in the literature. The relative prevalence of these two species is not yet fully known but it is clear that in most parts of the world E. dispar is easily the more common of the two. The two species are morphologically identical and differentiating between the two relies on relatively sophisticated methods: isoenzyme, antigen and/or DNA analyses. Kits for the differentiation of E. histolytica and E. dispar directly from faecal samples are available commercially from only one source but others will be available soon."
See also this abstract found through PubMed:
Trop Doct. 2004 Jan;34(1):28-30.
The stool examination reports amoeba cysts: should you treat in the face of over diagnosis and lack of specificity of light microscopy? Lawson LL, Bailey JW, Beeching NJ, Gurgel RG, Cuevas LE.;; Zankli Medical Centre, Abuja, Nigeria.
"Amoebiasis is a common clinical diagnosis in tropical settings and clinicians continue to treat asymptomatic carriers diagnosed by light microscopy. A minority of carriers, however, are infected with Entamoeba histolytica and the remaining with the non-pathogenic Entamoeba dispar. We compared the diagnostic results of 298 asymptomatic residents of Aracaju, Brazil, obtained by different diagnostic methods, and ascertained their clinical symptoms, to highlight the implications for practitioners. Fifty-eight (19.4%) specimens were amoebae positive by microscopy. Of these, 38 (13%) were E. histolytica/E. dispar enzyme-linked immunosorbent assay (ELISA) positive and 4 (1%) E. histolytica ELISA positive. The frequent use of anti-amoebic treatment on the basis of non-specific symptoms and the findings of light microscopy tests is not justified. Methods for the specific diagnosis of E. histolytica infection for developing countries are urgently needed."
This may throw into question a great deal of what has been published about the prevalence and virulence of "true" E. histolytica infection. The cysts of E. histolytica and E. dispar are identical. Microscopic identification requires finding the trophozoite with red cells inside in the stool. Given that you now have two non-pathogenic forms, Entamoeba coli and Entamoeba dispar, you probably need to modify your hypothesis. This article may give you more details. It is written by the group that discovered E. dipar:
The carriage rate of Entamoeba coli varies widely across the globe.
-----citations from PubMed follows---------
Presse Med. 2004 Jun 19;33(11):707-9. PMID: 15257226
[Evolution in the prevalence of intestinal parasitosis in the Fort de France University Hospital (Martinique)] [Article in French]
(Endolimax nanus, Dientamoeba fragilis, Entamoeba coli) found in 1.88% of population
Rev Esp Salud Publica. 1997 Nov-Dec;71(6):547-52.
[Epidemiology of children’s intestinal parasitism in the Guadalquivir Valley, Spain]
Giardia lamblia (5.05%), Entamoeba coli (2.45%), Endolimax nana (1.61%), Entamoeba histolytica (0.31%), Entamoeba hartmanni (0.05%), Iodamoeba butschlii (0.05%)
Enferm Infecc Microbiol Clin. 1995 Oct;13(8):464-8.
[Prevalence of intestinal parasites in a student population]
Giardia lamblia (36.36%), Entamoeba coli (10.43%), Entamoeba hartmanni (2.02%), Endolimax nana (1.34%), Enterobius vermicularis (1.34%), Ascaris lumbricoides (0.67%) and Trichuris trichiura (0.67%).
West Afr J Med. 1992 Apr-Jun;11(2):106-11.
Studies in intestinal parasitic disease agents in stools of people in a rural area of Nigeria.
Entamoeba coli (19.0%) Necator americanus (17.0%); Entamoeba histolytica (4.7%); Schistosoma mansoni (3.0%); Giardia lamblia (2.3%); Trichuris trichuria (1.7%); Trichomonas hominis (1.0%); Ascaris lumbricoides (0.7%); Hymenolepsis nana (0.3%) Endolimax nana o. 3%); stercoralis (0.23%), and Iodamoeba butschlii (0.3%).
This article studied multiple infections anomg Kenyans and found a very high occurrence of polyparasitism. I don’t have access to that paper, but it might be worthwhile tracking it down through your university.
East Afr Med J. 1991 Feb;68(2):112-23.
Intestinal parasites in a rural community in Kenya: cross-sectional surveys with emphasis on prevalence, incidence, duration of infection, and polyparasitism.
"In the first survey, 81.4% of the sample was positive for at least one intestinal parasite and 78% was positive for intestinal protozoa. 72.7% of those infected had multiple infections. "
" Significant positive associations between parasite species were common at all surveys, especially among the amoebae."
Rev Inst Med Trop Sao Paulo. 1990 Nov-Dec;32(6):428-35.
Parasitological and serological studies on amoebiasis and other intestinal parasitic infections in the rural sector around Recife, northeast Brazil.
"485 inhabitants in 4 villages: "Schistosoma mansoni (82.1%), hookworm (80.2%) Trichuris trichiura (69.9%), Ascaris lumbricoides (61.9%) and Entamoeba coli (36.7%) infections were demonstrated. Test tube cultivation revealed that the most common species of hookworm in this region was Necator americanus (88.4%), and also that the prevalence of Strongyloides stercoralis was 5.8%. Three hundred and thirty-four sera were serologically examined for amoebiasis by the gel diffusion precipitation test (GDP) and enzyme-linked immunosorbent assay (ELISA). No positive reaction was observed in all sera as examined by GDP, while 24 sera were positive by ELISA."
My interpretation is that the prevalence of Entamoeba histolytica is quite low in that population. Antibody detection in sera stays positive for years even after recovery.
Rev Inst Med Trop Sao Paulo. 1995 Jan-Feb;37(1):13-8. Related Articles, Links
Prevalence of intestinal parasitic infection in five farms in Holambra, SÃo Paulo, Brazil.
"…rates of 6 helminths and 7 protozoan species detected are as follows: 5.4% Ascaris lumbricoides; 8.6% Trichuris trichiura; 19.8% Necator americanus; 10.4% Strongyloides stercoralis; 1.4% Enterobius vermicularis; 0.9% Hymenolepis nana; 3.2% Entamoeba histolytica; 2.7% E. hartmanni; 9.9% E. coli; 14.0% Endolimax nana; 2.3% Iodamoeba butschlii; 10.4% Giardia lamblia; 37.8% Blastocystis hominis."
Cad Saude Publica. 2003 Mar-Apr;19(2):667-70. Epub 2003 May 15.
[Intestinal parasite infections in a semiarid area of Northeast Brazil: preliminary findings differ from expected prevalence rates] (article in Portugese)
"…Entamoeba coli (35.8%), Endolimax nana (13.6%), Hymenolepis nana (9.4%), and hookworm (9.4%)"