الفهرس 
Definition and characterization of lactic acid bacteria
Indicator pathogenic strainsOnly 14 pages are availabe for public view
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Abstract
Ahmed Mohamed Moawed Mabrouk. Use of Multifunctional
Bacterial Cultures for Improving Cheese Quality. Unpublished Ph.D.
Thesis, Department of Food Science, Faculty of Agriculture, Ain
Shams University, 2016.
In recent years, extensive searches have been carried out on the lactic
acid bacteria (LAB) for their potential use as probiotics and
biopreservatives in food and dairy industries. Nowadays, probiotic
bacteria play an important role in the majority of food supplementation
and extended shelf life of fermented foods. Isolation and identification of
LAB from fermented dairy products gained more attention allover the
world. So, total of 75 strains of LAB were isolated from local fermented
dairy products. These isolates were characterized morphologically,
physiologically and biochemically by using analytical profile index (API)
tests. Analytical profile index (API®50 CHL) system was used for
biochemical identification of Lactobacillus (Isolates grown on MRS agar)
, while analytical profile index (API®20 CH) strep system was used for
identification of Streptococcus (Isolates grown on M17 agar). From
seventy five isolated strains only eight strains were chosen as follows:
Lactobacillus rhamnosus (1 strain), Lactobacillus plantarum (1strain),
Lactobacillus pentosus (2 strains), Pediococcus pentosaceus (2 strains),
Lactobacillus brevis (1 strain) and Lactoccocus lactis ssp. Lactis (1
strain). These selected strains were proceeded to screen them in vitro for
probiotic traits (tolerance to different concentrations of bile salts, low pH
resistance, growth and viability % in different concentrations of NaCl,
phenotypic safety assessment and antibacterial activity against five
indicator pathogenic strains (Bacillus cereus, Staphylococcus aureus, and
Escherichia. coli 0157:H7, Listeria monocytogenes Type I and
Pseudomonas aeruginosa). Chosen strains were also examined for some
technological properties like acidifying activity, exopolysaccharides
(EPSs) production, survival in different concentrations of NaCl and milk
coagulation activity. The results revealed that all chosen strains were
observed to have good antagonistic activity against tested indicator
pathogens with differences in diameter of inhibition zone (mm).
Meanwhile, Pseudomonas aeruginosa and Listeria monocytogenes type I
were not inhibited by the extract of Pediococcus pentosaceus NRC AM 1
and Pediococcus pentosaceus NRC AM 4. According to the results of
technological and probiotic properties of examined eight lactic acid
strains, there were various behaviours in acidifying activity of
reconstituted sterile skim milk medium and most strains had good ability
to produce lactic acid and coagulate the milk. So, the good strains would
be a candidate in the dairy fermentation process as starter cultures,
whereas, the poor acidifier strains can be used as probiotic or adjunct
cultures. The viability of selected isolated strains grown in MRS broth
supplemented with different concentrations of NaCl were significantly
affected (P<0.05) and all viabilities gradually decreased by increasing
NaCl concentration. Only three strains Lb. pentosus NRC AM8 strain Lb.
pentosus NRC AM5 and Lb. plantarum NRC AM7 were able to resist the
concentration of 10 % NaCl. from the obtained results in the screening of
producing exopolysaccharides (EPSs), many isolated strains showed
ability to produce and synthesize EPS in vitro. Furthermore, all the tested
strains were able to grow and survive well in MRS supplemented with
different concentrations of bile salts, and the high resistant ability was
observed with the strains Pediococcus pentosaceus NRC AM 4 and
Lactococcus lactis ssp. lactis NRC AM3 respectively. All tested strains
were significantly affected (P<0.05) by exposure to pH 2 for 5 hours. On
the other hand, no blood haemolysis (B-hemolysis) was detected for any
tested strains. from this part, only four strains e.g. Lactobacillus brevis
NRC AM2, Pediococcus pentosaceus NRC AM4, Lactobacillus
rhamnosus NRC AM6, Lactobacillus pentosus NRC AM8 were selected
and confirmed by using 16S rRNA gene. The results confirmed that
Lactobacillus brevis NRC AM2 was identified as Lactobacillus casei
NRC AM2, Pediococcus pentosaceus NRC AM4 and Lactobacillus
rhamnosus NRC AM6 were identified as the same and Lactobacillus
pentosus NRC AM8 was identified as Pediococcus acidilactici NRC
AM8. The confirmed strains were interesting for using in dairy products.
Probiotic low salt soft cheese was manufactured by using one of the
selected strains e.g. Lactobacillus casei NRC AM2, Pediococcus
pentosaceus NRC AM4, Lactobacillus rhamnosus NRC AM6,
Pediococcus acidilactici NRC AM8. The control treatment was
manufactured without adding any starter culture. The cheese treatments
were manufactured from heat treated cow’s milk (80o C for 5 min.). The
resultant cheese treatments were evaluated for chemical, bacteriological
and organoleptic properties during refrigeration storage period at 7o C for
28 days. Statistical analysis showed that the type of starter and/or time of
storage period significantly affected the cheese analysis and sensory
evaluation of all cheese treatments. The treatment containing the strain of
Lactobacillus rhamnosus NRC AM6 had the highest score and
acceptability among all probiotic low salt cheese treatments.
Key Words:
Lactic acid bacteria, multifunctional bacterial cultures,
biochemical identification, antibacterial activity, indicator
strains, 16S rRNA gene and low salt soft cheese.