The major interactions between two organisms in a mixed culture are : Competition Neutralism Mutualism Commensalism Amensalism Prey-predator

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1 1 Introduction Major classes of interaction in mixed cultures Simple models describing mixed-cultures interactions Mixed culture in nature Industrial utilization of mixed cultures 2 1

2 The dynamic of mixed cultures are important consideration in some commercial fermentations. Many food fermentations, such as cheese manufacture, depend on multiple interacting species. The biological treatment of waste water relies on an undefined complex mixture of microorganisms. In all natural environments, cells exist in potentially mixed populations. Many populations with recombinant DNA behave as mixed-culturesystems. ( recombinants are discussed in chapter 14) 3 The major interactions between two organisms in a mixed culture are : Competition Neutralism Mutualism Commensalism Amensalism Prey-predator 4 2

3 Competition is an indirect interaction between two populations that has negative effect on batch. In competition, each population competes for the same substrate. Two populations or microorganisms with similar nutrient requirements usually compete for a number of common, nutrients when grown together. 5 Neutralism is an interaction where neither population is affected by the presence of the other. That is,there is no change in the growth rate of either organism due to presence of the other. Neutralism relatively rare. EXAMPLE : yogurt starter strains of Streptococcus and Lactobacillus in chemostat. Neutralism may occur in special environments where each species consumes different limiting substrates and neither species is affected by the end products of the other. 6 3

4 Mutualism and Protocooperation are more common than neutralism and may involve different mechanisms. In both cases, the presence of each population has a positive effect on the other. For mutualism, the interaction is essential to the survival of both species. In protocooperation, the interaction is non essential. 7 One mechanism is the mutual exchange of required substances or the removal of toxic end products by each organism. EXAMPLE : growth of phenylalanine-requiring strain of Lactobacillus and a folic acid-requiring strain of Streptococcus in a mixed culture. NOTE : symbiosis and mutualism are not the same. Symbiosis implies a relationship when two organisms live together. A symbiotic relationship may be mutualistic, but it may be also neutralistic, parasitic, commensalistic and so on. 8 4

5 Commensalism is an interaction in which one population is positively affected by the presence of the other. However, the second population is not affected by the presence of the first population. Two common mechanisms are the following: 1. The second population produces a required nutrient or growth factor for the first population. 2. The second population removes a substance from the medium that is toxic to the first population. 9 EXAMPLE (first type ) : production of H 2 S by Desulfovibrio ( through the reduction of SO 4 2- ), which is used as an energy source by sulfur bacteria. 10 5

6 EXAMPLE ( second type ) : removal of Lactic acid by the fungus Geotrichium candidum, which allows the growth of Streptococcus lactis. This interaction is utilized in cheese making using S.lactic. Lactic acid produced by S.lactis inhibits the growth of the bacteria. The fungus metabolizes Lactic acid and improves the growth condition for S.lactic. 11 Amensalism is the opposite of commensalism. In amensalism, population A is negatively affected by the presence of the other population ( B ). However, population B is not affected by the presence of population A. 12 6

7 Two common mechanisms are the following : 1. Population B produces a toxic substance that inhibits the growth of population A. 2. Population B removes essential nutrients from the media, thus negatively affecting the growth of population A. EXAMPLE ( first type ) : production of antibiotics by certain molds to inhibit the growth of others. 13 Predation and Parasitism are interaction in which one population benefits at the expense of the other. These two interactions are distinguished by the relative size of organisms involved. Predation involves the ingestion of prey by the predator organism. EXAMPLE : ingestion of bacteria by protozoa. 14 7

8 In parasitism, the host, which is usually the larger organism, is damaged by the parasite. The parasite benefits from utilization of nutrients from the host. EXAMPLE : destruction of microorganisms by microphages. In an open system, where predator-prey interactions take place, the population of the predator and prey do not necessarily reach steady state, but can oscillate at certain dilution rates. 15 Depending on dilution rate, and feed substrate concentration, the oscillations may be sustained or damped or may not exist. Finally, note that these interactions can, and often do, exist in combination. 16 8

9 17 Writing the appropriate equations to describe mixed population follows the principles was discussed in chapter 6. In each case, a balance must be written for each species (organism, rate-limiting substrate, or product), and these balances will be the same as we have used previously. 18 9

10 EXAMPLE 1 : Consider a case of mutualistic growth in a chemostat. How would you write the appropriate equations for this system? Determine the effects of the addition of competition to mutualism. (1) (2) (3) (4) (5) 19 (6) (7) (8(a)) (8(b)) (9) 20 10

11 EXAMPLE 2: Consider the growth of protozoa (predator)on bacteria(prey) in a chemostat. Write appropriate equations for this system. (10) (11) (12 ) 21 A classical model that describes oscillations in a prey-predator system is the Lotka-Volterra model, in which growth rates are expressed by the following equations: (13) (14) 22 11

12 The first term on eq.13 describes the growth of prey on substrate, and the second consumption of prey by predators. The first and second terms in eq.14 describe the death of predator in the absence of prey and the growth of predator on prey. 23 Y p/b is the yield of predator on prey (g/g) μ' b is the specific growth rate of prey on a soluble substrate (h -1 ) μ' p is the growth rate of predator on prey (1/g-h) k' d is the specific death rate of the predator (h -1 ) In steady state : dx b /dt and dx p /dt = 0 Under these conditions : X bf =k' d /μ' p X pf = μ' b Y p/b /μ' p 24 12

13 By defining dimensionless variables such as: (15) We can express eqs.13 and 14 in terms of X b and X P. (16) (17) 25 Equations 16 and 17 with the initial conditions of X b (0) =X b0 and X P (0) = X p0 The relationship between X b and X P can be determined by dividing eq.16 by 17. (18) 26 13

14 Integration of eq.18yields : (19(a)) (19(b)) Where K is an integration constant that is function of the initial population size. 27 The Lotka- Volterra model considers the exponential growth of prey species in the absence of predator and neglects the utilization of substrate by prey species according to Monod form

15 Mixed cultures of organisms are common in natural ecological system. Microorganisms are involved in the natural cycles of most elements (e.g., carbon, nitrogen, oxygen, and sulfur.) Specified diagram of the carbon cycle. Also shown (dashed lines)is the major component of the oxygen cycle, which is closely linked to the cycle of carbon

16 4.2.The global nitrogen cycle: Simplified schematic of the cycles of matter in an anaerobic environment: 32 16

17 5.1. Cheese : Defined mixed microbial populations are commonly used in cheese making. Cheeses of various types are produced by inoculating pasteurized fresh milk with appropriate lactic acid organisms. The bacteria used for lactic acid production are various species of Streptococcus and Lactobacillus in a mixed culture. Other organisms are used to develop flavor and aroma. Among these are Brevibacterium linens, Propionibacterium shermanii, Leoconostoc sp., and Streptococcus diacetilactis Whiskey: Lactic acid bacteria are also used whiskey manufacture. Lactobacillus added to the yeast reduces ph and, therefore, the chance of contamination. Lactobacillus also contributes to the flavor and aroma of whiskey. A favorable interaction between yeast and lactic acid exists in ginger-beer fermentation

18 5.3.Waste-water treatments process: The utilization of undefined mixed microbial cultures in waste-treatment processes is typical and unavoidable. Waste-water treatment constitutes one of the largest-scale uses of bioprocesses. Mixed cultures are also utilized in the anaerobic digestion of waste materials. Cellulase producers, acid formers, and methane producers are typical organisms involved in the anaerobic digestion of cellulosic wastes. 35 The Symba process was developed in Sweden for treating starchy wastes, particularly those from potato processing. This process utilizes Endomycopsis fubuligera for amylase production and yeast, Candida utilis, for the utilization of sugar molecules produced from the hydrolysis of starch. Corn and pea wastes are also treated by a mixed culture of Trichoderma viride and Geotrichium sp.t.viride produces cellulase to break down cellulose into reduced sugar molecules produced from the hydrolysis of starch

19 A mixed culture of Candida lipolytica and Candida tropicalis has been grown on hydrocarbons, n-paraffins, or gas oil for single-cell protein(scp) production purposes in both laboratory and pilot-scale production. Certain methane-utilizing bacteria such as Pseudomonas oxidize methane to methanol. However, Pseudomonas is inhibited by the end product, methanol. 37 Inclusion methanol-utilizing bacteria such as Hyphomicrobium into the growth medium eliminate the problem of methanol inhibition. This relationship is mutualistic in the sense that Pseudomonas supplies carbon source (CH 3 OH) for Hyphomicrobium, and Hyphomicrobium removes the growth inhibitor (methanol) of Pseudomonas

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