Triangulación de la información

The intrinsic and extrinsic factors control bacterial growth in food, but little was known about the molecular mechanism of bacterial behavior in these conditions until recently. Bacterial quorum sensing (QS) is thought to play a major role in the bacterial response to a changing environment in food. QS is described as a bacterial cell-to-cell communication system to develop a strategy for the bacterial population to adapt to the harsh environment in food or the host.12 _{QS is cell-density dependent, that is, when bacteria reach a certain cell number,}

they produce small diffusible molecules, termed autoinducers (AIs) as signaling molecules that are used for cell-to-cell communication during growth. For example, under stressful con- ditions, such as nutrient starvation, low water activity, high temperatures, antimicrobial com- pounds, acid, or other microbes, bacterial AIs bind to receptors and activate gene expression to alter behavior (i.e., adaptation to harsh environment) and morphological characteristics (i.e., biofilm formation, sporulation). There are four major categories of AIs: (i) autoinducer-1 (AI-1), for example, N-acyl homoserine lactone (Figure 6.2) is produced by Gram-negative bacteria and is used for intraspecies communication; (ii) autoinducer-2 (AI-2) is produced by both Gram-positive and Gram-negative bacteria and is used as a universal signaling molecule; (iii) autoinducer-3 (AI-3) is a signaling molecule used by enterohaemorrhagic Escherichia

coli during infection; and (iv) autoinducing peptides (AIPs) that are produced and used by

Gram-positive bacteria. AI-1 and AI-2 have been found in many different food systems, including milk, meat, and vegetables, when these foods are associated with pseudomonads,

Enterobactericeae, and lactic acid bacteria.

QS has been recognized as a key regulatory system in both food safety and food spoilage. In pathogenic bacteria, such as Salmonella, Escherichia coli O157:H7, and Staphylococcus aureus, QS is involved in virulence gene expression, promoting increased pathogen survival and colonization in the host.13 _{QS is also a regulatory spoilage mechanism.}12 _{Food spoilage is characterized by}

Factors Influencing Microbial Growth in Food  ◾  71

excessive microbial growth rendering food undesirable for consumption (see Chapter 19). Foods of animal origin are rich in protein, which is degraded by proteolytic enzymes produced by pseu- domonads, Enterobactericeae, and lactic acid bacteria, rendering spoiled food. Small molecules, such as glucose, lactate, and free amino acids, regulate growth of these microorganisms in these foods. Spoilage of fruits and vegetables depends on the pectinolytic activity (enzymes involved are pectin lyases, pectate lyase, polygalacturonase, pectin methylesterage) of members of the

Pseudomonadaceae and Enterobactericeae families resulting in enzymatic browning, off-flavor, and

texture breakdown. Production of these enzymes is regulated by QS; thus chemicals (i.e., garlic- extract plant-derived p-coumaric acid) that inhibit QS can be used to control bacterial growth in foods. Another strategy to control pathogen or spoilage bacterial growth in food would be to add QS compounds in foods to create a false sense indicating there are already too many bacteria pres- ent in food; thus growth can be ceased.12

Conclusion

The physical and chemical environments control microbial growth within the growth range mainly by influencing their metabolic process associated with synthesis of energy and cellular components. Beyond the growth range, these factors, either individually or in combination, can be used to control microbial growth and even to destroy them. Actual growth is accomplished through the metabolism of various nutrients present in a food. The processes by which the food nutrients are transported inside the microbial cells and then metabolized to produce energy, cel- lular molecules, and byproducts are briefly discussed in Chapter 8.

Adaptation survival Sporulation Biofilm formation Autoinducer (AI) Bacteria Virulence gene expression

N-acyl homoserine lactone N O O H R Spoiled bell pepper

Figure 6.2 (See color insert.) Bacterial quorum sensing network and adaptation strategy on food. Bacteria secrete an autoinducer (Ai), such as N-acyl homoserine lactone, to communicate with others in response to a changing food environment.

72  ◾  Fundamental Food Microbiology QUESTIONS

1. List the intrinsic and extrinsic factors necessary for growth of microorganisms in a food. 2. What are the major nutrients in food that are metabolized by the microorganisms? List the

major groups of carbohydrates present in foods. List the carbohydrates in milk and meat, the pentoses in plant foods, and an oligosaccharide in plant foods.

3. Discuss how bacteria are able to metabolize large molecules of carbohydrates, proteins, and lipids. How do molds differ from bacteria in the metabolism of these molecules?

4. Discuss the importance of antimicrobials in foods that can adversely affect microbial growth. 5. Define AW and explain the desorption and adsorption processes of moisture in a food.

Discuss the importance of AW in microbial growth. How do halophilic, osmophilic, and

xerophilic microorganisms differ in minimum AW needed for growth?

6. Define pH and discuss factors that influence the pH of a food. Discuss the role of pH on microbial growth. How does a bacterial cell maintain a high intracellular pH (6.0) while growing in a low-pH (5.0) environment? Give examples of an aciduric bacterium and an acid-tolerant bacterium.

7. Define redox potential and discuss how it influences microbial growth in a food. How can microorganisms be grouped on the basis of their growth capabilities at different redox poten- tials and oxygen availabilities?

8. How are microorganisms grouped on the basis of their temperature of growth and survival? Discuss the significance of psychrotrophic and thermoduric microorganisms in the process- ing and refrigerated storage of foods.

9. What is quorum sensing? How does quorum sensing affect bacterial growth in foods?

References

1. Lengeler, J.W., Drews, G., and Schlegel, H.G., Biology of the Prokaryotes, Blackwell Science, New York, 1999, pp. 80, 110.

2. Potter, N.N., Food Science, 2nd ed., AVI Publishing, Westport, CT, 1973, p. 36.

3. Conner, D.E., Naturally occurring compounds, in Antimicrobials in Foods, 2nd ed., Davidson, P.M., and Branen, A.L., Eds., Marcel Dekker, New York, 1993, p. 441.

4. Sperber, W.H., Influence of water activity of foodborne bacteria: A review, J. Food Prot., 46, 142, 1983. 5. Troller, J.A., Water relations to foodborne bacterial pathogens: An update, J. Food Prot., 49, 656, 1986. 6. Beuchat, L.R., Influence of water activity on growth, metabolic activities and survival of yeasts, J. Food

Prot., 46, 135, 1983.

7. Christian, J.H.B., Reduced water activity, in Microbial Ecology of Foods, Vol. 1, Silliker, J.H., Ed., Academic Press, New York, 1980, p. 70.

8. Corlett, D.A., Jr. and Brown, M.H., pH and acidity, in Microbial Ecology of Foods, Vol. 1, Silliker, J.H., Ed., Academic Press, New York, 1980, p. 92.

9. Baird-Parker, A.C., Organic acids, in Microbial Ecology of Foods, Vol. 1, Silliker, J.H., Ed., Academic Press, New York, 1980, p. 126.

10. Brown, M.H. and Emberger, O., Oxidation reduction potential, in Microbial Ecology of Foods, Vol. 1, Silliker, J.H., Ed., Academic Press, New York, 1980, p. 112.

11. Olson, J.C., Jr. and Nottingham, P.M., Temperature, in Microbial Ecology of Foods, Vol. 1, Silliker, J.H., Ed., Academic Press, New York, 1980, p. 1.

12. Skandamis, P.N. and Nychas, G.-J.E., Quorum sensing in the context of food microbiology, Appl. Environ. Microbiol., 78, 5473–5482, 2012.

13. Antunes, L.C.M., Ferreira, R.B.R., Buckner, M.M.C., and Finlay, B.B., Quorum sensing in bacterial virulence, Microbiology, 156, 2271–2282, 2010.

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Chapter 7

Microbial Attachments