In other bibliometric studies [19, 22] in which the document type is reported, the reviews are in second place after the articles. In our study, although letters represent the second type of document with the highest percentage of publications, only articles and reviews are among the 10 most cited documents on polymyxin resistance. An underlying explanation is that The Lancet Infectious Diseases and to a lesser extent other journals (Antimicrobial Agents and Chemotherapy, International Journal of Antimicrobial Agents and Journal of Antimicrobial Chemotherapy) accept research for publication under the category of research letter as a form of rapid communication, especially on relevant topics. An example of this is the most cited letter (173 citations) in the period 2009–2018 that belongs to Falgenhauer L. et al.  published in 2016 in The Lancet Infectious Diseases and which deals with the mcr-1 gene that confers resistance to colistin in carbapenemase-producing gram-negative bacteria and beta-lactamases that spread in Germany.
In 2016, there was a peak in the number of citations, exactly 8,033 citations, this is probably due to the great impact that the publication dealing with the paradigm shift had on the transfer mechanism of colistin resistance, which was considered only as a rare chromosomal mutation up to that time. Liu Y-Y et al.  demonstrated plasmid-mediated colistin resistance thanks to the mobilized colistin resistance (mcr-1) gene identified in pigs in China. Interestingly, in the same year another gene called mcr-2 was isolated in Eschericihia coli (E.coli) in Belgium . In the following years other mcr genes were identified: in 2017 the mcr-3 gene in E.coli in China , mcr-4 in E.coli and Salmonella enterica serovar Typhimurium in three countries (Italy, Spain and Belgium) , mcr-5 in E.coli and Salmonella Paratyphi B in Germany , and mcr-6 in Moraxella pluranimalium in the United Kingdom ; in 2018 the mcr-7.1 and mcr-8 gene, both in Klebsiella pneumoniae in China [35, 36]; in 2019 the mcr-9 gene in Salmonella enterica serovar Typhimurium in the USA ; and to date, the mcr-10 gene was isolated in Enterobacter roggenkampii in China . This has contributed to the continuous growth of the scientific literature relevant to polymyxin resistance, although they did not cause the same impact after 2016, as citations decreased (Fig. 1).
As in our study, other bibliometric studies reported that the USA ranked first in the amount of scientific production in resistance to other antimicrobials [19, 20, 22, 23]. Likewise, the USA also obtained the first place in those studies that analyzed the scientific production of a certain drug, such as tramadol , aspirin  or clavulanic acid . Given the large number of documents from the USA (n = 146), it is logical to expect them to obtain the highest number of citations (6,513). However, the UK achieved a significant number of citations (3,587) with fewer documents (n = 44). This may be due to their great international collaboration (79.5%), being 7.8 times more cited than the world average (Table 3). In addition, it has three publications within the 10 most cited documents on polymyxin resistance (Table 2: rank 1, 7, and 9).
Nordmann P. and Poirel L., who were the authors with the highest number of documents in polymyxin resistance, also have the highest number of documents in carbapenem resistance according to the bibliometric study carried out by Sweileh W. et al. . Both authors (Nordmann P. and Poirel L.) have as affiliation the University of Fribourg and present multiple co-authorships in their researchs, of which the most cited (126 citations) deals with the role of the MgrB protein as a source of resistance to colistin in a series of Klebsiella pneumoniae . Nonetheless, the most influential authors on polymyxin resistance were Doi Y. (USA) and Wang Y., (China) with a FWCI of 15.71 and 16.07, respectively. The fact that both authors are co-authors of the most cited article would explain their high FWCI (see Table 2). It is important to note that this is not their only publication in the top 10 (Table 2: rank 7).
As for the 10 institutions with the highest scientific production, none of them belongs to the USA. Univeristy of Zhejiang (China) was the institution whose publications caused the greatest impact with 2,956 citations and 79 authors, with Feng Y. being the most productive author with this affiliation (see Table 4). Despite this, the University of Fribourg (Switzerland) with only 16 authors obtained the highest number of documents (n = 44). It is also important to point out that, the University of São Paulo (Brazil), while not having international collaboration, ranked 9th among the most productive institutions (Table 5). Its important national collaboration (68.2%) is noteworthy, being its greatest contribution on dissemination of the mcr-1 gene through samples collected in E. coli and other enterobacteria, evidencing an emerging resistance to colistin in the South American continent since 2012 .
Although Antimicrobial Agents and Chemotherapy, Journal of Antimicrobial Chemotherapy and Eurosurveillance were the journals in which the 10 most cited documents were published three times, twice and twice, respectively, The Lancet Infectious Diseases with only 26 documents was 16.91 times more cited than the world average, achieving 137.9 citations/article, which triples the number of citations/article obtained by the journal with the largest number of documents in our study (Antimicrobial Agents and Chemotherapy). However, Antimicrobial Agents and Chemotherapy is the only one that has remained among the 5 journals with the highest scientific production in other bibliometric studies, ranking first on carbapenem resistance  and antifungal triazole resistance , second in antimalarial drug resistance , and third in antimicrobial resistance among uropathogens .
Finally, there are some limitations and strengths in our research. First, like other bibliometric studies, some results may have been missing due to publication in non-indexed journals in Scopus. However, this is the first study on polymyxin resistance applying bibliometric indicators. Second, it was not possible to discern between articles related to humans and animals. Despite this, the relationship is closer than it seems because colistin is used as a growth promoter in the veterinary field . Furthermore, horizontal transmission of mcr genes occurs through multi-resistance plasmids from animals, humans, and retail meat [43, 44].