الفهرس 
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Abstract
Fish is one of the most important food that human eat, because it contain animal protein and important amino acids. It also a good source of necessary fat (omega3), vitamins and minerals. It also contain small amount of salts.
One hundred and twenty random samples of Tilapia niloticus, Mugil cephalus, Carp and Sardine were collected from markets at Kafrelsheikh city for bacteriological examination to determine the quality of fish. The results indicated that the mean values of the aerobic plate counts were 6.8×105 ± 1.3 ×105, 3.07×105± 5.2×104, 4.2×105± 6.8×104and 3.6×105± 6× 104 cfu/g in Tilapia niloticus, Mugil cephalus, Carp and Sardine, respectively. The mean values of psychrophilic bacterial counts were 4.3×105± 9.6×104, 2.8×105±5.9×104, 2.1×105±3.7×104 and 9.2×105± 2.5×105 cfu/g in Tilapia niloticus, Mugil cephalus, Carp and Sardine, respectively.
On the other hand, the mean values of Enterobacteriaceae counts were 9.2×103±1.1×103, 1.2×104±2.1×103, 3.8×104 ±1.1×104 and 1.02×104 ±2.3×103 cfu/g in Tilapia niloticus, Mugil cephalus, Carp and Sardine, respectively. The mean values of coliform counts were 5.3×103 ±8.5×102, 5.1×103 ±6.9×102, 5.9×103 ±1.4×103 and9.4×103 ± 2.3×103 cfu/g in Tilapia niloticus, Mugil cephalus, Carp and Sardine, respectively.
Treatments of tilapia fish by different organic acids as acetic acid and citric acid in different concentrations (1%, 2% and 3% of each acid) enhance the quality of fish and increase its shelf life. The shelf life was 3 days in control samples (untreated samples), but in groups treated by acetic acid1%, 2% were 15 days and 18 days in group treated by acetic acid 3%. While in groups treated by citric acid 1%, 2% were 12 days and 15 days in group treated by citric acid 3%.
The reduction percent of aerobic plate counts of examined fish samples dipped in different concentration of some organic acids at zero time was ranged from 83.990% to 88.177% in groups treated by acetic acid and 55.911% to 92.857% in groups of citric acid and reached to99.999% in group of acetic acid 3% at 18th day and 99.990%in group of citric acid at 15th day.
While the reduction percent of psychrophilic bacterial counts of examined fish samples dipped in different concentration of some organic acids at zero time was ranged from 60% to 82.222% in groups treated by acetic acid and 51.852% to 80% in groups of citric acid and reached to99.998% in group of acetic acid 3% at 18th day and 99.989%in group of citric acid at 15th day.
The reduction percent of Enterobacteriacea counts of examined fish samples dipped in different concentration of some organic acids at zero time was ranged from 89.032% to 94.516% in groups treated by acetic acid and 95.968% to 96.290% in groups of citric acid and reached to99.979% in group of acetic acid 3% at 18th day and 99.931 %in group of citric acid at 15th day.
The reduction percent of coliform counts of examined fish samples dipped in different concentration of some organic acids at zero time was ranged from 47.222% to 77.778% in groups treated by acetic acid and 78.704% to 85.185% in groups of citric acid and reached to 99.990% in group of acetic acid 3% at 18th day and 99.882% in group of citric acid at 15th day.