Competitive exclusion

The concept of competitive exclusion involves the purposeful inoculation of animals with a desirable gut microflora. This is not a nutrient-based supplementation. However, an oral route of inoculation is used, and the potential of competitive exclusion for improving food safety is substantial. Thus, a brief discussion of this principle is provided. 

Episodes of food-bome illness associated with the consumption of animal food products are often caused by bacteria. Salmonella and Campy- 

The inoculation of chickens with non-pathogenic cultures of bacteria has been reported to reduce Salmonella populations in chickens (Nurmi and Rantala, 1973 Wierup et al, 1988 Rehe, 1991). Probiotic administration is quite similar in approach to competitive exclusion in that a specific bacterial culture, often lactic acid bacteria, is administered to live birds. The two procedures may also accomplish the same goals with respect to pathogen reduction. However, a tenet of probiotic administration has been that it improves animal growth (Jernigan et al., 1985). 

The exact mechanism by which competitive exclusion operates is unknown. Mead and Barrow (1990) have suggested that the exclusion of Salmonella spp. may be by a variety of ways including competition for nutrients and/or adhesion sites on intestinal mucosa, production of inhibitory substances and alteration of pH. Inhibitory substances may include bacteriocins, hydrogen peroxide and/or organic acids (Juven et al., 1991). 

It is important to note that in their extensive and successful study, Wierup et al. (1988) analyzed the livers and caeca of chickens for Salmonella spp. Studies in which adult broilers are processed under commercial conditions are required, to determine if Salmonella occurrence on carcasses is reduced. The need to maintain strict hygiene under processing operations will continue to be necessary to ensure minimal contamination. 

---

Hume M.E., Byrd III J.A., Stanker L.H. and Ziprin R.L. (1998). Reduction of caecal Listeria monocytogenes in leghorn chicks following treatment with a competitive exclusion culture . Lett Appl Microbiol, 26, 432-436. 

Nisbet D. (2002). Defined competitive exclusion cultures in the prevention of enteropathogen colonisation in poultry and swine . Antonie van Leeuwenhoek, 81, 481 186. 

Flighly stable, uniform environments with abundant resources allow the dominance of particularly competitive species, whereas moderate stresses may decrease the likelihood of competitive exclusion. 

Chapelle, F. H. and D. R. Lovley, 1992, Competitive exclusion of sulfate reduction by Fe(III)-reducing bacteria a mechanism for producing discrete zones of high-iron ground water. Ground Water 30, 29-36. 

Chauviere, G., Coconnier, M. H., Kemeis, S., Dareuille-Michaud, A., Joly, B., and Servin, A. L. (1992). Competitive exclusion of diarrheagenic Escherichia coli (ETEC) from human entrocyte-Uke Caco-2 cells by heat-killed Lactobacillus. FEMS Microbiol. Lett. 91, 213-218. 

Microorganisms approved for feed use under the EU regulations comprise Enterococcus faecium (in various forms) and Saccharomyces ceremsiae. Their use as probiotics (as an alternative to antibiotics) is based on the principle of promoting the growth of lactobacilli and reducing the numbers of enteropathogenic bacteria in the gut. Sometimes this principle is referred to as competitive exclusion. This aspect will be addressed in more detail in Chapter 7 (this volume). 

The phenomenon by which the normal GI microflora protects the host against invading pathogens is termed competitive exclusion. It implies the prevention of entry or establishment of one bacterial population into the GI tract by a competing bacterial population already occupying potential attachment sites. To be able to succeed, the latter population must be better suited to establish or maintain itself in that environment or must produce compounds inhibitory to its competitors. 

Two processes are involved in speciation. Firstly, the two populations must evolve sufficient ecological differences that they do not suffer from competitive exclusion. In the case of metal-tolerant ecotypes, this condition has been fulfilled. Secondly, they must evolve sufficient barriers to gene exchange that the two gene pools are effectively isolated, and will evolve independently. There are a number of ways in which this could occur, and the first stages of this have been observed in a number of studies of metal-tolerant plants. 

Shaw and Bell (1991) examined this effect in the case of competition between radiocaesium and the K+ and NH4+ ions during root uptake by wheat (Triticum aestivum). These authors formalised the observed relationships in terms of classical Michaelis-Menten kinetics which necessitates the assumption that each of these ions is taken up by identical sites associated with the root plasmalemma. Lembrechts et al. (1990) found a similar negative and non-linear relationship between the concentration of Ca either in soil or in solution culture and the degree of radiostrontium uptake by lettuce Lactuca saliva). The principle of competitive exclusion of a radionuclide by an ion analogue may be exploited, with varying degrees of success, as a post-con-... 

FIGURE 1.1 Probiotics may protect against infection by pathogens through (1) Direct antagonism via bacteriocin production. (2) Immunomodulation via immune cell (T-cell, Dendritic cell) activation. (3) Improvement of epithelial barrier function and competitive exclusion via induction of mucus and blocking of epithelial binding receptors. 

Fukata, T., Sasai, K., Miyamoto, T., and Baba, E., Inhibitory effects of competitive exclusion and fructooligosaccharides, singly and in combination, on Salmonella colonization of chicks, J. Food Prot., 62, 229-233, 1999. 

Mulder, R.W.A.W., Havenaar, R., and Huis in t Veld, J.H.J., Intervention strategies the use of probiotics and competitive exclusion microflora against contamination with pathogens in pigs and poultry, in Probiotics 2 Applications and Practical Aspects, Fuller, R., Ed., Chapman Hall, London, 1997, pp. 187-207. 

The contextuality of meaning may be called a principle , for it is neither a brute fact nor a law of nature. But exactly what is meant by a principle is hard to specify. We can give some familiar examples of course. In ecology there is the well-known competitive exclusion principle , which explains why organisms occupying exactly the same niche cannot coexist for more than a brief period of time. In logic we all use, whether we know it or not, the principle of contradiction , which states that two propositions that really contradict each other cannot both be true. And since, by implication, at least one of them must be false, we justify the kind of hypothetico-deductive scientific... 

Certain strains of bacteria can competitively exclude other strains. Lindow (117) found that Ice (ice nucleation-active) populations of Pseudomonas syringae on corn plants Zea mays L.) were reduced by prior colonization of the leaves by Ice strains (ice nuleation-deficient). Interestingly, the total bacterial population was not affected by the prior inoculation of Ice strains (117). For these reasons, Lindow (117) concluded that the antagonism between these strains resulted from competitive exclusion of new arrivals to an occupied niche rather than competitive displacement. As the carrying capacity of leaf surfaces for bacteria is low relative to other plant habitats, particularly roots (101), preemptive colonization by one strain may be sufficient to reduce the population size of similar strains. The importance of competition for space among naturally colonizing microbes remains unknown. 

The grossly different values for the two closely related Vaccinium species which usually do not occur next to each other hence are probably results of different metal acquisition strategies rather than due to the competitive exclusion principle both plant species differ in spatial distribution rather than in take-up of resources (for example, both are rich in manganese while the differences can be seen in pairs of other metals). 

Theorem 5.1 is an example of the principle of competitive exclusion only one competitor can survive on a single resource. Many of the well-known... 

Figure 7.1. Example of Monod-type uptake (or growth) functions a /j(S) dominates/jlS) for all values of S, and competitive exclusion holds b /,(S) and fi S) cross exactly at the value of D, producing coexistence, the knife-edge phenomenon c / (S) and fi(S) cross, and competitive exclusion holds.

In the previous chapter it was shown that the simple chemostat produces competitive exclusion. It could be argued that the result was due to the two-dimensional nature of the limiting problem (and the applicability of the Poincare-Bendixson theorem) or that this was a result of the particular type of dynamics produced by the Michaelis-Menten hypothesis on the functional response. This last point was the focus of some controversy at one time, inducing the proposal of alternative responses. In this chapter it will be shown that neither additional populations nor the replacement of the Michaelis-Menten hypothesis by a monotone (or even nonmonotone) uptake function is sufficient to produce coexistence of the competitors in a chemostat. This illustrates the robustness of the results of Chapter 1. It will also be shown that the introduction of differing death rates (replacing the parameter D by D, in the equations) does not change the competitive exclusion result. 

Our main result here states that competitive exclusion holds for n competitors in a chemostat provided each competitor possesses a monotone uptake function. The proof follows [AM]. 

The biological conclusion is, of course, that differing removal rates do not alter competitive exclusion in the chemostat. One anticipates that a similar conclusion is true if the Michaelis-Menten dynamics is replaced by the general monotone term f,(S) used in Section 3. However, the Liapunov calculations depend on this form and the general question is still unresolved. 

It has been shown that competitive exclusion - that is, the extinction of all but one competitor - holds regardless of the number of competitors or the specific monotone functional response. If one restricts attention to the Michaelis-Menten functional response, then competitive exclusion has been shown even in the case of population-specific removal rates. 

Case (i) of the theorem corresponds to competitive exclusion Xj is the winner. The second case represents a new phenomenon - namely, the possibility that too much nutrient can lead to the washout of all populations. Both the competitive exclusion outcome (represented by ,) and washout (represented by Eq) have significant likelihood which one occurs depends on the initial conditions. That washout of all populations (including Ar,) is possible in case (ii), for some set of initial conditions, is not difficult to anticipate. If S(0) > then f S)—D < 0 for all /, so all populations decrease initially. If the populations are initially small then they may be washed out of the chemostat before they are able to lower the nutrient concentration to favorable levels. 

---