**3.8. Propionibacteria**

334 Olive Germplasm – The Olive Cultivation, Table Olive and Olive Oil Industry in Italy

anaerobically at 37°C for 18-48 hours.

**3.7.** *Listeria monocytogenes*

were reported in 2010.

(Beumer & Curtis, 2003).

**Figure 5.** *L. monocytogenes* colonies on ALOA medium.

extract as sources of H2S with ferric ammonium citrate as the indicator. In any case, subsequent confirmation tests such as motility, reduction of nitrate, lactose fermentation, gelatin liquefaction (Labbe, 2001) should be necessary. The plates are incubated

Listeria species are ubiquitous organisms widely distributed in the environment, especially in plant matter and soil. The microorganism is established as an important foodborne pathogen, which can grow at high salt concentration (up to 10% of NaCl) and at refrigerated temperatures. A study carried out by the U.S. Food and Drug Administration, U.S. Department of Agriculture (USDA, 2001), indicated that vegetables are able to support *L. monocytogenes* growth and, after a few years, Caggia and co-workers (2004) demonstrated that *L. monocytogenes* can survive and grow in green table olives. As indicated by the most recent scientific report of the European Food Safety Authority (2012), the number of listeriosis cases in humans in the EU slightly decreased, and 1,601 confirmed human cases

The minimum infective dose of *L. monocytogenes* has not yet been established and many authorities require that the organism must be absent in 25 g of product. This has led to the development of methods following the sequence of pre-enrichment, selective enrichment and diagnostic plating. The whole procedure may take about five days. Then, presumptive positive results need to be confirmed adding further time to complete the examination

Most of the isolation media differentiate *Listeria* spp. by means of aesculin hydrolysis which, in the presence of iron, forms a black phenolic compound. According to the USDA method Propionibacteria are often associated with food spoilage. The "zapateria" spoilage, which can occur in brined olives, is characterized by a malodorous fermentation due to propionic acid produced by certain species of *Propionibacterium* (Jay, 2000), alone or together with *Clostridium* spp.. The propionibacteria may be considered as marker of the end of shelf life of table olives, because the growth of these bacteria cause an increase in pH values creating conditions for the growth of spoilage or pathogen bacteria and the arising of off-odors (Plastourgos & Vaughn, 1957).

Unfortunately, propionibacteria are difficult to isolate because they grow very slowly on solid media. The complex medium usually used is not able to suppress competing organisms. This medium is Sodium Lactate agar (1% typticase, 1% yeast extract, 1% sodium lactate, 0.025% dipotassium phosphate, 1.5% agar) in which the propionibacteria growth appears in 5-7 days at 32°C under anaerobic or microaerophilic conditions (Richter & Vedamuthu, 2001).
