Several indicators suggest that sea bream colonization by bacteria belonging to Moraxella species might be increasing in Mediterranean mariculture plants, and that presence of these bacteria in internal organs of farmed fish might be linked to slow growth, reduced wellness, and higher susceptibility to other diseases. In this study, the prevalence of internal organ positivity to Moraxella sp. was found to be of 1.35% on almost 1,000 fish examined. Moraxella sp. colonization was detected throughout the year, in all locations examined, and at different temperatures, both in floating offshore cages and lagoons (Table 1). In general, fish colonization was not associated to evident symptoms, with the exception of a severe exophtalmus observed upon eye positivity. Eye symptoms are consistent with infection by Moraxella sp. in other animal species and in humans. Therefore, Moraxella sp. might be causing ocular damages (i.e. exerting direct pathogenic effects) also in fish, although presence of other underlying conditions could not be ruled out completely.
Many diseases of fish have a multifactorial origin, and might stem from presence of an aetiological agent combined with environmental stresses. As such, Moraxella sp. might be considered an opportunistic bacterial pathogen, which could cause illness in "stressed" fish. Indeed, colonization of internal organs might impair the physiological tissue functions and render fish more vulnerable to the environment or to the occurrence of infections by other pathogens [3, 4]. Moreover, as reported in other organisms, Moraxella sp. might act as an opportunistic pathogen in fish under acute or chronic stressful conditions, with increased susceptibility to other infectious diseases. In both circumstances, the identification of biochemical markers enabling identification of an early host response to asymptomatic infections by Moraxella sp. and/or other opportunistic Gram-negative pathogens appears of considerable interest. The availability of a preclinical marker would be welcome for detecting colonization and for containing bacterial infection outbreaks before they spread to the whole cage or plant, especially when uncontrolled, wild fish are used for restocking.
The molecular approach used in this study offered us the opportunity to investigate the protein expression profile of healthy, bacteria-free kidneys, and to evaluate the changes that renal tissue undergoes when Gram-negative bacteria are present. Higher relevance was given to proteins with a marked upregulation in the infected kidney, in order to pinpoint proteins with potential as markers of Gram-negative infections, which could be used as tools to quickly detect the menace of an outbreak in the farming plant and enable intervention within short time frames, avoiding loss of fish.
In Moraxella-positive sea bream kidney tissue, seven out of ten proteins showing a statistically significant upregulation were mitochondrial enzymes (Table 2, bold). This was an interesting finding, since mitochondrial proteins are central to various metabolic activities and are key regulators of apoptosis. Notably, disturbance of mitochondrial proteins is often associated with disease .
Interestingly, other non-mitochondrial proteins significantly upregulated in Moraxella- positive kidney tissue were peroxiredoxins and S-adenosyl-homocysteine hydrolase, together with indications about upregulation of antiquitin, Warm acclimation-related protein 65, transferrin, glutathione S-transferase, carbonic anhydrase, and Cu/Zn superoxide dismutase, many of which are known to be related to cellular responses to oxidative stress, infection, inflammation, or programmed cell death processes. Transferrin was also found to be significantly upregulated in rainbow trout serum following intraperitoneal inflammation and LPS injection .
The number of fish used to gather proteomic data in this study was limited, and variations in expression levels of mitochondrial proteins might as well have been induced by other metabolic perturbations, or be dependent on underlying alterations due to non-infectious factors. This notwithstanding, the value of the enzymatic markers proposed in this study is enhanced and reinforced by the fact that our findings, observed in naturally occurring infections by Moraxella sp., are consistent with the data reported by Roher and coworkers  following experimental administration of LPS. These authors investigated the physiological consequences of administrating LPS to rainbow trout. Important changes in metabolic, mitochondrial, and structural genes were observed according to tissue metabolism. In aerobic tissues, that obtain energy mainly from oxidative phosphorylation, LPS provoked marked changes in representative mitochondrial genes, whereas in anaerobic tissues major expression changes were observed in glycolytic enzymes. The findings reported by these authors reinforce the considerations about the potential detrimental effects of internal organ colonization by Gram-negative bacteria, including opportunistic pathogens, on growth and metabolic processes of fish.