Abstract
Background:
Aquaculture sustainability is increasingly threatened by environmental stressors, particularly deteriorating water quality, which can enhance the susceptibility of fish to opportunistic bacterial infections. Members of the Enterobacter cloacae complex are widely distributed in aquatic environments and may act as opportunistic pathogens under unfavorable conditions.

Aim:
This study aimed to isolate and phenotypically identify Enterobacter cloacae complex from diseased Grey Mullet (Mugil cephalus) in Sousa Harbor, Libya, and to evaluate its pathogenicity under different water quality conditions.

Methods:
Bacterial isolates were obtained from moribund fish and identified using morphological, biochemical, and API 20E analyses. Healthy fish were experimentally challenged using a bathing method under three controlled water quality conditions (good, moderate, and poor). Infection and mortality rates were recorded, and statistical analyses including Fisher’s exact test and logistic regression were performed. Antibiotic susceptibility testing was also conducted.

Results:
Infection and mortality rates increased significantly with deteriorating water quality. Fish under poor conditions showed the highest infection (80%) and mortality (40%) rates, compared to 20% infection and 10% mortality under good conditions. Logistic regression indicated that fish in poor water quality were approximately 16 times more likely to become infected (OR = 16.0, P = 0.018). The isolate exhibited resistance to multiple β-lactam antibiotics.

Conclusion:
The findings confirm that Enterobacter cloacae complex acts as an opportunistic pathogen whose virulence is strongly influenced by environmental conditions. Poor water quality significantly exacerbates disease severity, highlighting the critical importance of water quality management in aquaculture systems as a primary preventive strategy.

Key words: Enterobacter cloacae; Mugil cephalus; Sousa; Water quality; Antibiotic resistance.