As shown in Table 1 SLI-biofilm was more frequently produced at both 25°C and 37°C in Acinetobacter Genospecies 3, Acinetobacter Genospecies 13TU, and A. baumannii than in A. johnsonii, A. lwoffii, and A. radioresistens (p < 0.05 for all comparisons). Thus, these results might explain, at least in part, the marked persistence of the three former species in hospitals and their involvement in nosocomial infections. Moreover, in a previous study (using the same collection of A. baumannii clinical isolates), Rodriguez-Baño et al. showed that 63% of A. baumannii clinical isolates formed SLI-biofilm at 37°C. Based on these results, our study revealed that the rates of SLI-biofilm formation obtained for Acinetobacter Genospecies 13TU and A. baumannii were similar, as well as their variation related to the temperature (higher at 25°C than at 37°C). Likewise, ALI-biofilm was significantly more frequent at both 25°C and 37°C in Acinetobacter Genospecies 13TU and A. baumannii than in A. johnsonii, A. lwoffii, A. radioresistens, and Acinetobacter Genospecies 3 (p < 0.05). Overall, this similar behaviour in biofilm formation between A. baumannii and Acinetobacter G13TU is coherent with the fact that these two species are the most commonly found in the hospital.
Our results showed (see Table 1) that ALI-biofilm was mainly formed by the members of the ACB-complex. Although A. junii had some capacity to form this characteristic biofilm, the number of isolates studied was too small to draw conclusions. By contrast, as a member of the ACB-complex, A. calcoaceticus has shown a different pattern at 37°C with a complete absence of ALI-biofilm formation together with a reduced ability to form SLI-biofilm, both of which could be explained by the fact that this species' natural habitat is the environment where lower temperatures usually prevail.
Finally, as biofilm formation generally predominated at 25°C rather than 37°C, this mechanism could explain the observed persistence of the members of the A. baumannii group in the inanimate hospital environment. Previous studies have already reported that the members of the A. baumannii group and especially A. baumannii survive desiccation better than other Acinetobacter spp., comparing the survival rate of A. baumannii to those obtained for Staphylococcus aureus[7, 17]. In addition, in a recent study, Wisplinghoff et al showed that all disinfectants tested inhibited the growth of A. baumannii which suggests that these microorganisms in their planktonic state are susceptible to most disinfectants. As already demonstrated for other bacterial species, extracellular polymeric substances from the biofilm matrix play an important role in the resistance and tolerance to dehydration , suggesting that biofilm formation contributes to the ability of A. baumannii, and possibly the other members of the A. baumannii group, to survive better in the hospital environment and to resist the action of disinfectants.
For A. baumannii, SLI-biofilm has already been described in several reports [20–22] and it has been linked to some device-associated infections . Indeed, de Breij et al have recently compared SLI-biofilm formation by the members of the ACB complex and concluded that there was no difference between clinically relevant and less-relevant Acinetobacter strains and species; in the same way, no temperature related differences were found for biofilm formation. Our results slightly differ from theirs possibly due to the higher number of strains and species analysed. They suggested a reduced biofilm formation for the Acinetobacter Genospecies 13TU although only 3 isolates had been studied; by contrast, after studying 60 isolates, our results clearly show that the behaviour for biofilm formation of this species is highly related to A. baumannii. On the other hand, although we have only studied 10 A. calcoaceticus isolates, the results also indicate an association between biofilm and temperature in this non-pathogenic species, a tendency that can be observed in most of the analysed species. The comparison with other non-pathogenic species outside the ACB-complex has shown a relevant difference for biofilm formation that was even increased when looking at the ALI-biofilm formation, an ability that to our knowledge has never been described in clinical isolates of the Acinetobacter spp.
In summary, the members of the A. baumannii group have a higher ability to form SLI-and ALI-biofilm than other less clinically related species. Nevertheless, A. calcoaceticus, the environmental representative of the ACB-complex, showed a reduced biofilm formation at 37°C when compared to the other members of this complex. This feature could be connected to the higher colonization rate of patients by pathogenic Acinetobacter species (mainly A. baumannii and Acinetobacter Genospecie 13TU), and probably contributing to the increased risk of clinical infection.