The battle against Listeria continues

Lactic acid bacteria are effective and are actively used to combat dangerous food-borne bacteria. Against one of the most dangerous and persistent bacteria – Listeria monocytogenes – the bacteriocin Sakacin P appears to be a promising candidate. However, use of it can cause bacteria to become resistant.

In his doctoral research, Girum Tadesse Tessema has studied the effect of compounds produced by lactic acid bacteria on Listeria monocytogenes. He has focused particularly on the bacteriocin Skacin P and on organic acids such as lactic acid and acetic acid.

“New” types of bacteriocins

Bacteriocins are small proteins that are produced by lactic acid bacteria, among other things, and that have a lethal or inhibiting effect on other bacteria. Bacteriocins have potential as additives that could increase food safety. There are many types and groups of bacteriocins. At present the only bacteriocin that can be approved as an additive to make food safer is nisin. But in the fight against dangerous food-borne bacteria, we can now see the opportunity to approve “new” types of bacteriocins in order to find more protectors. One of the most interesting of these is Sakacin P, which can combat Listeria monocytogenes.

The results of doctoral candidate Girum Tadesse Tessema’s research at Nofima are extremely relevant in this context, in particular the fact that the molecular analysis has been much more complex than in previous studies.

“The range of Listeria monocytogenes bacteria is greater and the studies more in line with reality, because the Listeria bacteria have been exposed only to a single application of bacteriocin, which will also be the case if it is used as a food additive,” explains Girum Tadesse Tessema.

The results of the studies show that Sakacin P destroys a large proportion of Listeria bacteria, but that some bacteria can survive. These become resistant by creating mutations (changes in the genetic material). These so-called mutants can also be divided into different groups. In the next phase, comparisons between mutants and original bacteria show that there are greater variations between these groups than was previously thought.

A research team from Nofima, consisting of Kristine Naterstad, Lars Axelsson and Trond Møretrø, has been supervising Tessema during his doctoral research.

“This research has given decision makers a broader and better basis to work from. We know more now about the uncertainties relating to the use of Sakacin P in combating Listeria monocytogenes. Whatever measures are taken against undesirable bacteria, such as Listeria, problems can arise with more or less resistant strains. The results do not rule out the use of Sakacin P and other bacteriocins from the same group, but further research is important,” says Naterstad.

Less aggressive mutants

There are great differences in the genetic response of mutants compared with the original Listeria monocytogenes bacteria. Analysis shows for example that the groups react differently to certain types of stimuli.

“The discovery of these differences has been made with the aid of so-called microarray technology and has led to a new classification that is more detailed and more stable than that which has been used to date. The advantage of using microarray is that the expressions of all genes in a subject can be studied simultaneously,” says Girum Tadesse Tessema.

One of the interesting findings in these analyses has been the indications that the mutants appear to be somewhat less virulent – less aggressive in causing disease – than the original mother strain. It is thus possible that this leads to a kind of endeavour among Listeria bacteria to become resistant and that this affects how “capable” they are in other contexts.

Acetic acid has the best effect

Organic acids are currently actively used to combat Listeria and they are found in all fermented food. As well as studying Sakacin P, Girum Tadesse Tessema has investigated how Listeria reacts to two acids that are produced by lactic acid bacteria, lactic acid and acetic acid, and compared these with hydrochloric acid.

“Many genes are affected and they react differently to different acids. There is a greater similarity in the bacteria’s genetic reactions when acid stress is caused by acetic acid and lactic acid than with hydrochloric acid. This is because acetic acid and lactic acid have some structural similarities and also belong to the group of organic acids. Acetic and lactic acids are more effective against Listeria monocytogenes than is hydrochloric acid, and of these in turn, acetic acid has the best effect. Acid stress can however make Listeria bacteria more dangerous. It is therefore necessary to add a sufficient quantity of acid, so that all the dangerous bacteria die,” says Girum Tadesse Tessema.

Enough but not too much

The challenge for food producers lies in adding enough acid to kill the dangerous bacteria, but not using more than is necessary. Adding too much acid has a negative effect on the taste.

“I have analysed genetic response in Listeria using microarray technology. All the tests have been carried out in test tubes. A natural continuation of such research would be to perform such tests in the food itself, to see whether the effects we see in the test tube compare with what happens when Listeria is found in food. These effects can then be connected to see how Listeria actually survives in food in different conditions. This could give us some useful new information that would be relevant for food safety,” concludes Girum Tadesse Tessema.

Food safety and quality  

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