8 - Listeria monocytogenes - A lethal foodborne bacterium and the composition of cell membranes PDF

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Listeria monocytogenes - A lethal foodborne bacterium and the composition of cell membranes

Around the late 1970s/early 1980s a new threat to public health came to be recognised. The organism at the centre of this threat was the bacterium Listeria monocytogenes which turned out to be foodborne. The disease induced by L. monocytogenes is known as ‘listeriosis’ and is associated with a high case mortality that ranges from 20 to 50 %. Pregnant women are at particular risk as listerioisis can lead to spontaneous abortion. The table below shows international foodborne-associated disease outbreaks of listeriosis, 1980-2005.

Source: Swaminathan, B. & Gerner-Smidt, P. (2007)

The temporal pattern/occurrence of listeriosis remains unpredictable and subject to a number of uncontrollable factors as evidenced by the figure below which shows incidences of listeriosis in Austria, 1997-2008.

Source: Allerberger & Wagner (2009)

A number of factors have contributed to the emergence of this new pathogen and these relate primarily to changes in patterns of food preparation and consumption in western industrialised countries. In particular has been the trend towards increased consumption of convenience meals. The principal characteristic of L. monocytogenes that enabled it to emerge as a human pathogen is its ability to grow over an extraordinarily wide range of temperatures; this ranges from about 1 to 43º C – wider than any other organism. This means that L. monocytogenes is able to grow at refrigeration temperatures as well as at ambient ones. Another feature is its tolerance to very high salt concentrations. Organisms can be classified according to their optimal growth temperatures and how the categories psychrophile, mesophile and thermophile were defined. Listeria monocytogenes is referred to as a ‘psychrotroph.’ Psychrophilic and psychrotrophic microorganisms share the ability to grow at low temperatures, but psychrotrophic microorganisms have a maximum temperature for growth above 20º C and are widespread in natural environments and in foods. Psychrophilic microorganisms on the other hand have a maximum temperature for growth at 20º C or below and are restricted to permanently cold habitats. How does L. monocytogenes achieve this remarkable ability to grow over such a wide temperature range? The cells of all living organisms are bounded by a membrane which is comprised of a double layer of lipid molecules – the ‘lipid bilayer’ as it is generally referred to. The membrane performs a number of functions, one of which is control over the transport of molecules into and out of the cell. Imbedded in the membrane are a number of proteins some of which are enzymes. Some of these function to actively transport molecules into and out of the cells. This is illustrated below:

Source: Brock, The Biology of Micro-organisms

The activity of these proteins is dependent on the maintenance of membrane fluidity. As the temperature drops the membrane ‘freezes’ and its fluidity is reduced. Bacteria have the ability to change the composition of their membranes in order to maintain membrane fluidity. L. monocytogenes has an extended ability to do this, and this explains its ability to grow over a particularly wide temperature range. We shall now examine the composition of cell membranes. As was mentioned above, cell membranes are formed of lipid bi-layers. ‘Lipid’ is the generic term used for fats, oils and waxes. Lipids are insoluble in water but soluble in organics such as chloroform. They serve the cells in a variety of ways which includes storage of metabolic energy and as carriers of biological signals. The lipids of biological membranes are characterised by the presence of fatty acids. Fatty Acids Fatty acids are long chain hydrocarbons containing a terminal carboxyl (COOH) group. The hydrocarbons may be saturated or partially unsaturated.! The former contain a terminal methyl (-CH3) group, a variable number of methylene groups (-CH2) and a terminal carboxyl group. They range in length from C14 to C22 and may contain branches. Unsaturated fatty acids contain one (monoenoic) or more (polyenoic) double bonds, however, fatty acids with more than one double bond are not found in bacteria. Moreover the double bonds are invariably of the cis- conformation. There are two shorthand systems commonly used for naming fatty acids and these are illustrated below: Example 1 Palmitic Acid C16 : 0 i.e. 16 carbons and no unsaturation 16 1 CH3(CH2)14C00H ↑ ↑ Ω-carbon α-carbon

! Example 2 Oleic Acid C18 : 1Δ9 i.e. 18 carbons: 1 double bond at C-9

18 9 1 CH3(CH2)7CH=CH (CH2)7C00H

Example 3 Eicosopentaenoic Acid (EPA) C20 : 5Δ5, 8, 11, 14, 17

i.e. 20 carbons: 5 double bonds at C-5, 8, 11, 14, 17

! Lipid formation occurs when fatty acids are esterified with OH-containing compounds, the commonest of which is the trihydric alcohol glycerol. A fatty acid may be esterified to any one of the three –OH groups in which case a monoacylglycerol is formed. The esterification of two and three fatty acids produces diacyl- and triacylglycerols respectively. Overall, the reaction is as follows:-

A so-called ‘simple’ triacylglyceride (or triglyceride) is obtained if R = R’ = R’’

Triacylglycerides containing 2 different fatty acids can exist in 6 different forms

1

B

A

A

A

B

B

2

B

A

B

B

A

A

3

A

B

A

B

A

B

Phosphoglycerides (Phospholipids) These are important constituents of cell membranes and are ultimately derived from phosphatidic acid at the C-3 position on the glycerol molecule.

•

Chiral centre at C2 (L-form shown)!

•

Orientation of the molecule separates polar and non-polar parts:

Phospholipids form micelles in water

!

At air-water interfaces they orient themselves as shown below

!

But to return to L. monocytogenes and its emergence as a threat to food; this bacterium has a number of strategies for maintaining optimal fluidity of its membrane. As mentioned above, this entails changing the composition of its membrane. The fluidity of bacterial membranes is influenced by the proportion and type of fatty acids present. For example, in the presence of cisunsaturated fatty acids the packing forces are weaker than with fully saturated fatty acids and individual lipids retain a high liquid mobility at low temperature.

Another strategy employed by L. monocytogenes is that of chain branching.

The full range of strategies employed by L. monocytogenes is illustrated below:...