Acinetobacter baumannii has emerged as a highly troublesome pathogen for many institutions globally. As a consequence of its immense ability to acquire or upregulate antibiotic drug resistance determinants, it has justifiably been propelled to the forefront of scientific attention. Apart from its predilection for the seriously ill within intensive care units, A. baumannii has more recently caused a range of infectious syndromes in military personnel injured in the Iraq and Afghanistan conflicts. This review details the significant advances that have been made in our understanding of this remarkable organism over the last 10 years, including current taxonomy and species identification, issues with susceptibility testing, mechanisms of antibiotic resistance, global epidemiology, clinical impact of infection, host-pathogen interactions, and infection control and therapeutic considerations.

Acinetobacter baumannii: Emergence of a Successful Pathogen

β-lactams have a long history in the treatment of infectious diseases, though their use has been and continues to be confounded by the development of resistance in target organisms. β-lactamases, particularly in Gram-negative pathogens, are a major determinant of this resistance, although alterations in the β-lactam targets, the penicillin-binding proteins (PBPs), are also important, especially in Gram-positive pathogens. Mechanisms for the efflux and/or exclusion of these agents also contribute, though often in conjunction these other two. Approaches for overcoming these resistance mechanisms include the development of novel β-lactamase-stable β-lactams, β-lactamase inhibitors to be employed with existing β-lactams, β-lactam compounds that bind strongly to low-affinity PBPs and agents that potentiate the activity of existing β-lactams against low-affinity PBP-producing organisms.

Resistance to β-lactam antibiotics

Sphingomonadaceae (n = 86) were isolated from a drinking water treatment plant (n = 6), tap water (n = 55), cup fillers for dental chairs (n = 21), and a water demineralization filter (n = 4). The bacterial isolates were identified based on analysis of the 16S rRNA gene sequence, and intraspecies variation was assessed on the basis of atpD gene sequence analysis. The isolates were identified as members of the genera Sphingomonas (n = 27), Sphingobium (n = 28), Novosphingobium (n = 12), Sphingopyxis (n = 7), and Blastomonas (n = 12). The patterns of susceptibility to five classes of antibiotics were analyzed and compared for the different sites of isolation and taxonomic groups. Colistin resistance was observed to be intrinsic (92%). The highest antibiotic resistance prevalence values were observed in members of the genera Sphingomonas and Sphingobium and for beta-lactams, ciprofloxacin, and cotrimoxazole. In tap water and in water from dental chairs, antibiotic resistance was more prevalent than in the other samples, mainly due to the predominance of isolates of the genera Sphingomonas and Sphingobium. These two genera presented distinct patterns of association with antibiotic resistance, suggesting different paths of resistance development. Antibiotic resistance patterns were often related to the species rather than to the site or strain, suggesting the importance of vertical resistance transmission in these bacteria. This is the first study demonstrating that members of the family Sphingomonadaceae are potential reservoirs of antibiotic resistance in drinking water.

Diversity and antibiotic resistance patterns of Sphingomonadaceae isolates from drinking water.

Virulence and antimicrobial resistance in clinical Enterococcus faecium.

The genus Myroides comprises aerobic, yellow-pigmented, non-motile, non-fermenting gram-negative rods formerly classified as Flavobacterium odoratum. Members of the genus are widely distributed in the environment, especially in water, and usually behave as low-grade opportunistic pathogens, having been found to cause urinary tract infection, endocarditis, ventriculitis, and cutaneous infections in severely immunocompromised patients. We report a case of soft tissue infection, septic shock, and pneumonia due to M. odoratimimus in an immunocompetent male. To our knowledge, this is the first description of life-threatening infection caused by this organism in an immunocompetent host. We have also reviewed the medical literature on the genus Myroides.

https://link.springer.com/content/pdf/10.1007%2Fs15010-010-0077-1.pdf

Septic shock, pneumonia, and soft tissue infection due to Myroides odoratimimus: report of a case and review of Myroides infections.

Microbiological cultures from 229 patients seeking medical advice in Stockholm after the tsunami catastrophe of December 2004 were analysed at the Clinical Microbiology Laboratory, Karolinska University Hospital, Stockholm, Sweden. Gram-negative bacilli were the most common findings from wound cultures. Common human pathogens such as Escherichia coli, Proteus species, Klebsiella spp., and Pseudomonas aeruginosa were isolated. More rare species of Gram-negative bacilli, e.g. Myroides odoratus, Sphingomonas paucimobilis and Bergeyella zoohelcum were also isolated. Resistance towards ordinary antibiotics was more extensive compared to our Swedish reference material for Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis and Acinetobacter spp., but not for Pseudomonas aeruginosa, probably reflecting that the resistant isolates were nosocomially acquired in Asia.

Gram-negative bacteria from patients seeking medical advice in Stockholm after the tsunami catastrophe

Enterococcus faecium has six penicillin-binding proteins (PBP), where PBP5 seems to be the main target for beta-lactam antibiotics. The PBP profiles of three imipenem-resistant, ampicillin-sensitive E. faecium strains, isolated from the same patient, were studied using biotinylated ampicillin and chemiluminescence detection. Imipenem resistance in these strains was found to be associated with hyperproduction of PBP5 compared to the ampicillin- and imipenem-susceptible strain ATCC 19434. PBP5 in the imipenem-resistant strains (S1, B2) exhibited a selectively decreased affinity for imipenem. An 854 bp DNA fragment, corresponding to the penicillin-binding domain of pbp5fm, was studied in the resistant strains and the reference strain. Four amino acid substitutions were observed in the resistant strains compared to the susceptible one. The contribution of these substitutions to the increased production of PBP5 in these strains is unclear since the substitution was observed also in a strain without increased production of PBP5. Our results suggest that the moderate imipenem resistance observed in these strains is associated with increased production of PBP5 with relatively decreased affinity for imipenem, and that evolution of imipenem resistance in E. faecium is dinstinct from that of the other beta-lactams such as ampicillin.

Mechanisms of resistance to imipenem in imipenem-resistant, ampicillin-sensitive Enterococcus faecium.

Microbiological cultures from 229 patients seeking medical advise in Stockholm after the tsunami catastrophe December 2004 were analysed at the Clinical microbiology laboratory, Karolinska University Hospital, Stockholm, Sweden. Gram-negative rods were the most common findings from wound cultures. Common human pathogens as Escherichia coli, Proteus species, Klebsiella spp, and Pseudomonas aeruginosa were isolated. However, more rare species of gram-negative rods were also isolated, e.g. Myroides odoratus, Sphingomonas paucimobilis and Bergeyella zoohelcum. Resistance towards ordinary antibiotics was higher compared to our Swedish reference material for Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis and Acinetobacter spp, but not for Pseudomonas aeruginosa. Possibly, this reflects that the resistant isolates were nosocomially acquired in Asia.

Christina Lundberg

Papers

Gram-negative bacteria from patients seeking medical advice in Stockholm after the tsunami catastrophe

Mechanisms of resistance to imipenem in imipenem-resistant, ampicillin-sensitive Enterococcus faecium.

[Rare bacteria species found in wounds of tsunami patients. Predominance of gram-negative rods, increased antibiotic resistance].