Low Occurrence of Acinetobacter baumannii in Gulls and Songbirds

Acinetobacter baumannii is a Gram-negative bacterium, which causes nosocomial infections worldwide. Due to its remarkable capability to develop multidrug resistance, we are running out of treatment options in many parts of the world (Murray et al. 2015). The World Health Organization (WHO), therefore, ranked carbapenem-resistant A. baumannii on the top of a priority list of antibiotic-resistant bacteria for the treatment of which new antibiotics are urgently needed (Tacconelli et al. 2018).

The natural reservoirs of A. baumannii are poorly defined. Many members of the genus Acinetobacter have ubiquitously spread bacteria found in soil, water, and other environmental sites (Afshinnekoo et al. 2015; Al Atrouni et al. 2016) but also as part of the human microbiome (Fyhrquist et al. 2014). A. baumannii has been isolated from different environmental sites and various animals (Eveillard et al. 2013; Rafei et al. 2015; Kittinger et al. 2017; Furlan et al. 2018). DNA-based studies suggest the rates of colonization of human head and body lice of up to around 50%, depending on the population (Kempf et al. 2012). However, an isolate from lice studied in detail was found to be phylogenetically distinct from clinical isolates and exhibited low virulence (Vallenet et al. 2008; Antunes et al. 2011). In a recent study, A. baumannii was isolated from white stork (Ciconia ciconia) nestlings in Poland at an average rate of 25% (Wilharm et al. 2017). Moreover, genome-based analyses revealed a multitude of distinct lineages and identified the relationship of some stork isolates to clinical strains. Given that A. baumannii was also isolated from poultry livestock, hatcheries, and meat (Martin and Jackel 2011; Lupo et al. 2014; Wilharm et al. 2017), we speculated about a general preference of A. baumannii for avian hosts. To challenge this hypothesis, we performed a culture-based screening of different free-living birds, the results of which are presented here.

In the course of a previously published study on the cultivable microbiota from the choana of free-living birds captured in Slovenia between September and December 2013 (Škraban et al. 2017), 25 choana samples were screened for the presence of A. baumannii as previously established for studies on white stork nestlings (Wilharm et al. 2017). None of the samples yielded A. baumannii isolates.

Of the 145 and 140 songbirds captured along the Baltic Sea during spring and autumn, respectively, in 2016, the European robin Erithacus rubecula was the dominating species (n = 210), followed by blackbird Turdus merula (n = 37), and blackcap Sylvia atricapilla (n = 11) (Table I). Choana and cloacal swabs were all found to be negative for A. baumannii.

The cloacal swabs sampled from black-headed gulls in 2017 (n = 187) were all negative for A. baumannii. Among the individuals sampled cloacally in 2018 (n = 554), two were positive for A. baumannii (from breeding colonies Koronowo and Przykona, respectively) (Table II). Sequencing of the bla _OXA-51-like gene intrinsic to A. baumannii revealed that the two isolates carried the OXA-71 and OXA-208 variant, respectively, of the class D β-lactamase OXA-51 family. Both isolates were susceptible to meropenem, imipenem colistin, tigecycline, gentamicin, tobramycin, amikacin, ciprofloxacin, and sulfamethoxazole-trimethoprim. Natural intrinsic resistance to chloramphenicol, penicillins (piperacillin, ampicillin), and cephalosporins (cefotaxime, cefoxitin) was observed for both isolates. In addition, eight isolates of the species Acinetobacter pittii were obtained from gull adults (breeding colonies Koronowo and Jankowo).

Acinetobacter baumannii has been isolated sporadically from different avian sources in the past (Ahmed et al. 2007; Muller et al. 2010; Zordan et al. 2011; Kempf et al. 2012; Rafei et al. 2015; Klotz et al. 2018). Additional studies providing evidence for the presence of A. baumannii in bird samples include a study on the rectal carriage of Gram-negative bacteria among 89 seabirds in rehabilitation centres in the USA. In this study, A. baumannii was isolated from 10% of birds, including gulls (Steele et al. 2005). However, the taxonomic classification of bacteria in that study was solely based on phenotypic traits, without the DNA-based corroboration. Even higher isolation rates of more than 30% were recently reported from 48 wild birds’ fecal samples collected in Nigeria (Dahiru and Enabulele 2015). Again, we cannot be certain about the taxonomic claim regarding these isolates due to the lack of genetic evidence. A DNA-based study on 73 pigeon droppings revealed the presence of A. baumannii in 5% of the samples from France and Algeria (Morakchi et al. 2017). Moreover, isolation of A. baumannii was reported from poultry hatcheries (Martin and Jackel 2011; Wilharm et al. 2017), poultry meat (Lupo et al. 2014; Carvalheira et al. 2017), and poultry livestock (Lupo et al. 2014). It has recently been demonstrated that 661 white stork nestlings studied in different regions of Poland over a period of four years were colonized at an average rate of 25% (Wilharm et al. 2017). However, systematic studies for the presence of A. baumannii are not available for other free-living birds leaving open the question of whether birds, in general, are favourable hosts for A. baumannii.

Here, for the first time, a large and diverse set of altogether 1,051 free-living birds was selectively screened for the presence of A. baumannii in cloacal samples (overall 1,026 samples) and tracheal samples (overall 310 samples), using the protocol established for studies on white stork nestlings (Wilharm et al. 2017). The extremely low prevalence of 0.3% in black-headed gulls and non-detection in 310 songbirds compared to 25% prevalence in white stork nestlings strongly argues against a general preference of A. baumannii for avian hosts. In line with this conclusion, no A. baumannii was isolated in a study on the aerobic cloacal and pharyngeal bacteria of 167 free-living birds in Germany (Stenkat et al. 2014). However, our study is limited by the opportunistic sampling design, affecting the species range, further causing potential biases due to specific behaviour and seasonality.

The two A. baumannii strains isolated from gulls were susceptible to clinically relevant antibiotics and exhibited only natural resistance to some penicillins, cephalosporins, and chloramphenicol. This is in full agreement with the resistance profiles previously described for the isolates from white stork nestlings (Wilharm et al. 2017). In conclusion, there is no evidence that these strains faced significant anthropogenic selection pressure in the past suggesting that the strains were acquired from natural habitats.

In summary, the data presented here indicate that there is no general association of A. baumannii with wild birds. Given the obvious risk of poultry livestock to serve as a vector for spreading of A. baumannii (Martin and Jackel 2011; Lupo et al. 2014; Wilharm et al. 2017), it is important to study poultry livestock as well as wild populations of chicken, geese, ducks, turkeys and related species globally to assess the risk potential associated with these populations and to complete our ecological understanding of pathogenic Acinetobacter species.