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2. LITERATURE REVIEW
2.1 Salmonella
2.1.1 Taxonomy and Characteristics
Salmonellae are motile, non-spore forming, Gram- negative, rod-shaped
bacteria that belong to the family Enterobacteriaceae. The molecular of the G + C Guanine and Cytosine content in their DNA is 50 53 Jay, 2000. There are
approximately 2,400 Salmonella serovars, which are classified under two species, S. enterica
and S. bongori Jay et al., 2005. Salmonella
grows on a large number of culture media, and produce visible colonies within 24 hours at about 37
° C. The optimum pH growth for this organism is
around neutrality 6.6 8.2 with values above 9 and below 4 are bacteriocidal. It grows at high water activity, below 0.94 growth is slowed and even inhibited Prost and
Riemann 1967. The organism cannot tolerate high salt concentrations; above 9 of a product composition salt concentration is considered bacteriocidal. Its temperature range
for growth is 4 to 45 °
C MacDonald, and Matney. 1963. With respect to destruction all Salmonella
are readily destroyed at milk pasteurization temperatures.
Table 1.0 Limits for the growth of Salmonella under otherwise optimal conditions,
adapted from International Commission on Microbiological Specifications for Foods 1996.
Parameter other conditions being optimal
Minimum Maximum
Temperature C 5.2
most serotypes
will not
grow at
7.0 46.2
pH 3.8 most serotypes will not grow below 4.5
9.5 Water activity
0.94 0.99
Source: Chris and Kyriades 2002.
The bacteria can catabolize D-glucose and other carbohydrates with the production of acid and gas. The organism species are oxidase negative and catalase
negative, grow on citrate as a sole carbon source, generally produce carbon sulfide, decarboxylase lysine and ornithine and do not hydrolyse urea. Many of these traits form
the presumptive basis for the biochemical identification of Salmonella isolated Doyle, Beuchat and Montville 2001.
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2.1.2 Gram Negative bacteria cell and pathogenic components
Gram negative bacteria possess several distinct surface layers that can enhance their pathogenicity. These layers are flagella, capsules and cell wall components. In
figure 1, general bacterial that is of pathogenic nature is portrayed.
Figure 1. General cell structure of bacteria.
Source: Fix 1997 2009.
Flagella are the organs of motility. Flagella are composed of flagellins proteins that make up the long filament. This filament is connected to a hook and rings that
anchor the flagella in the cell wall. In Gram-positive bacteria, there are two rings attached to the cytoplasmic membrane; in Gram-negative cells, an additional two rings
are found in the outer membrane. Flagella may be up to 20 µm in length. Some bacteria possess a single polar flagellum monotrichous, others have several polar flagella
lophotrichous, others have several flagella at each end of the cell amphitrichous, and still others have many flagella covering the entire cell surface peritrichious. Flagella
may serve as antigenic determinants e.g. the H antigens of Gram-negative enteric bacteria Fix, 1997 2009.
Capsules are types of surface layer composed primarily of high molecular weight polysaccharides. If the layer is strongly adhered to the cell wall, it is called a capsule; if
not, it is called a slime layer. These layers provide resistance to phagocytosis and serve as antigenic determinants, the Vi or K antigen. The production of capsules is genetically
and phenotypically controlled.
18 The cell wall is the basis for classification of bacteria according to the Gram
stain. Gram-positive bacteria have a thick layer of peptidoglycan external to the cytoplasmic membrane. In contrast, Gram-negative bacteria have a thin layer of
peptidoglycan located between the cytoplasmic membrane and a second membrane called the outer membrane. This region is known as the periplasmic space.
Peptidoglycan is a polymer of alternating N-acetylmuramic acid NAM and N- acetylglucosamine NAG. Long strands of this alternating polymer may be linked by L-
alanine, D-glutamic acid, L-lysine, D-alanine tetrapeptides to NAM. Gram-positive cells have a much more highly cross-linked peptidoglycan structure than Gram-negative cells.
Peptidoglycan is also the target of antimicrobial activity. For example, penicillins interfere with the enzymes involved in biosynthesis of peptidoglycan while lysozyme
physically cleaves the NAM-NAG bond. Lipopolysaccharides LPS are found only in Gram-negative bacteria. These structures are composed of lipid A, which binds the LPS
in the outer membrane and is itself the endotoxic portion of the molecule. The polysaccharide moiety appears on the cell surface, serving as an antigenic determinant
O antigen Wheelis 2007. Somatic O antigens in Gram negative bacteria are situated in the outer membrane
of the cell and are attached to lipids, commonly known as lipid A. Serological tests use somatic O to determine the bacterial serovars specifically being tested using antibodies
of presumed organisms suspected to be present to specifically get identified. The presence of an outer membrane and the possession of only few
peptidoglycan layers in the cell wall distinguish Gram-negative bacteria from Gram- positive ones figure 2. Lipids covalently linked to proteins called lipoproteins are the
molecules that bind the peptidoglycan to the outer membrane. The peptidoglycan is located in the periplasm, a space filled with fluid located between the plasma membrane
and the outer membrane. A high amount of degradative enzymes and transport proteins are found in the periplasm. Unlike Gram-positive cell walls, we cannot find teichoic
acids in the Gram-negative cell walls. In addition, the cell walls of Gram-negative bacteria are more prone to mechanical breakage because of the low amount of
peptidoglycan. Shagam 2006
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2.1.3 Salmonellosis
Kategori