Module 5. Microbes in spoilage of milk

Lesson 22

ROLE OF MICROBES IN SPOILAGE OF MILK - MICROBIAL INTERACTIONS

22.1 Introduction

Food spoilage is mainly caused by the growth of bacteria and yeast and moulds. The major requirements of microbial growth are the availability of nutrients as well as favourable growth conditions like temperature, water activity, presence or absence of oxygen, redox potential, pH value etc. While growing in an environment that has diversified population of microbes encounter different conditions that may be favourable or detrimental for the growth or proliferation. 

22.2 Microbial Interactions

Micro-organisms do not exist alone in nature, but in a matrix of other micro-organisms of many species. Thus, interaction is a rule of nature. When one or more types of micro-organisms reciprocate each other’s effect, it is called interaction. The evidence for such an interaction is very well established. Population of one species is different in absence and presence of a second species. Interaction between two different biological population can be classified according to whether both populations are unaffected by the interaction, one/ both populations benefit or one/ both population are adversely affected.

Microbial association in same environment can be naturalistic, antagonistic (negative) and symbiotic (positive).

22.2.1 Neutralism

Neutralism describes the relationship between two species that interact but do not affect each other. It describes interactions, where fitness of one species has absolutely no effect, whatsoever on that of other. True neutralism is unlikely and impossible to prove. When dealing with complex networks of interactions presented by ecosystems, one cannot assert positively that there is absolutely no competition between or benefit to either species. Since, true neutralism is rare or nonexistent; its usage is often extended to situations, where interactions are merely insignificant or negligible.

This type of association is most unlikely, as the two micro-organisms living in a close proximity are not affected by each other. This may exist between two micro-organisms, whose growth requirements are different and hence, affect neither kind as there will be no competition between these two for the nutrients.

22.2.2 Antagonistic association

When micro-organism adversely affects the environment of another micro-organism it is said to be antagonistic. Antibiosis is antagonistic association between two micro-organisms in which one is adversely affected e.g. production of antibiotic or inhibitory substances by one micro-organism that affect the growth or survival of another micro-organism. Lactic acid bacteria produce lactic acid that is inhibitory to spoilage organisms. In antagonistic interactions, one species benefits at the expense of another.

22.2.3 Symbiosis

The term symbiosis is used to describe degrees of close relationship between micro-organisms of different species. Sometimes, it is used only for cases, where both micro-organisms benefit, and sometimes it is used more generally to describe all varieties of relatively tight relationships, i.e. even parasitism, but not predation, for example a large number of bacteria are present in the intestinal tract. The bacteria act to break down foodstuffs, and then directly participate in the digestive process. As well, some of the intestinal bacteria produce products that are crucial to the health of the host. For example, some of the gut bacteria make vitamin k, vitamin B_12, biotin and riboflavin. These vitamins are important to host, but are not made by the host. The bacteria benefit by inhabiting an extremely hospitable environment. The natural activities and number of bacteria, also serve to protect the host from colonization by disease causing microorganisms. The importance of this type of symbiosis is exemplified by the adverse health effects to the host that can occur when the symbiotic balance is disturbed by antibiotic treatment given to the host.

22.2.3.1 Mutualism

Mutualism is an interaction between two or more species, where species derive a mutual benefit. Mutualism may be classified in terms of the closeness of association, the closest being symbiosis, which is often confused with mutualism. One or both species involved in the interaction may be obligate, meaning these cannot survive in the short or long term without other species.

However, in almost all cases, each micro-organism benefits from the association but the manner in which benefit is derived varies in the exchange of nutrients between two species. Association results in metabolic end products that are different from association as compared with sum of the products of separate species e.g. Proteus vulgaris ferment lactose and produce acid, Staphylococcus aureus ferment lactose and produce acid but together these produce gas and acid both.

22.2.3.2 Commensalism

It refers to a relationship between microorganisms in which one micro-organism benefits from the association but the other micro-organism is not affected. Host by its growth, affects the physiological environment in such a manner that commensal species is favoured or it occurs when one micro-organism takes benefits by interacting with another micro-organism by which the host is not affected e.g. facultative micro-organism grows and produces anaerobic conditions that favour growth of anaerobes. Growth of yeasts in sugar solutions reduces the concentration of sugar, thus permitting the growth of bacteria.

22.2.3.3 Synergism

Synergism, like mutualism, represents an association between two microbial populations in which both population benefit from each other, but it differs from the mutualism in that the association is not ‘obligatory’. Both synergistic populations of microbes are able to survive in their natural environment on their own.  Synergism allows microbial population to perform metabolic activities like synthesis of a product that neither population could produce alone e.g., bacterial synergism exists between a number of normal flora and a potential pathogens. This means one micro-organism is helping another to grow or survive. There are examples of a member of the normal flora supplying a vitamin or a growth factor that a pathogen needs to grow. This is called cross-feeding between microorganisms and is categorized under synergism.