Over the study period a total of 855 BCs obtained from patients suffering from a Gram-negative BSI were processed. Among these, 261 (30.5%) belonged to patients admitted in emergency and internal medicine departments, of which 90.4% (n=236) were reliably identified from bacterial pellet as EB species by MALDI-TOF MS analysis and included in the assessment of the fast-track workflow. Overall, polymicrobial BCs involving EB species were 12 (4.9%). Of these, ten BCs were not considered for study purpose due to unreliable MALDI-TOF identification score. The EB species mainly observed were Escherichia coli (68.2%; n=161), Klebsiella pneumoniae (18.2%; n=43), Enterobacter cloacae (4.2%; n=10), and Klebsiella aerogenes (3%; n=7).
The mean TtDP was 12.3 h (range 2–68). The mean TtR for conventional and fast-track workflow were 38 h (range 27–78) and 42 min (range 35–55), respectively.
Phenotypic characterization of EB species included in the study and likelihood of deduction of in vitro antibiotic susceptibility according to conventional culture-based results were reported in Table
2. Among
E. coli isolates, 20.5% and 79.5% were CTX-M-p and CTX-M-CA-np, respectively. No CA-p
E. coli were detected. CTX-M-p
E. coli were in vitro susceptible to TZP (84.8%), C/T (97%), CZA (100%), and AK (75.8%). CTX-M-CA-np
E. coli were largely in vitro susceptible to TZP (90.6%), CAZ (99.2%), FEP (100%), C/T (100%), CZA (100%), and AK (100%). Among AmpC/ESBL-np
E. coli isolates, 9.5% (
n=12) were resistant to TZP, whereas the only isolate resistant to CAZ was AmpC-p. Among
K. pneumoniae isolates, 23.3%, 20.9%, and 55.8% were CTX-M-p, CA-p, and CTX-M-CA-np, respectively. CTX-M-p
K. pneumoniae were in vitro susceptible to C/T (90%), CZA (100%), and AK (90%), whereas they showed susceptibility to TZP of 40%. All the CA-p
K. pneumoniae isolates were pheno-genotypically characterized as KPC-producers, showing susceptibility to CZA and AK of 88.9% and 44.4%, respectively. Analysis of the KPC gene sequence of the only CZA-resistant CA-p
K. pneumoniae isolate subsequently revealed to be a producer of KPC-31-carbapenemase harboring the mutation D179Y in the omega loop region of KPC-3 [
23]. CTX-M-CA-np
K. pneumoniae were in vitro susceptible to TZP (75%), CAZ (87.5%), FEP (100%), C/T (95.8%), CZA (100%), and AK (100%). Among AmpC/ESBL-np
K. pneumoniae, 19.1% (
n=4) were resistant to TZP, whereas among AmpC-p
K. pneumoniae, 33% (
n=1) and 66% (
n=2) were susceptible to TZP and C/T, respectively. All the
E. cloacae isolates were CTX-M-CA-np. They were in vitro susceptible to TZP (80%), CAZ (80%), FEP (80%), C/T (80%), CZA (100%), and AK (100%). All the AmpC/ESBL-p
E. cloacae (
n=2) were resistant to TZP, CAZ, FEP, and C/T. All the
K. aerogenes isolates were CTX-M-CA-np. They were largely in vitro susceptible to FEP (85.7%), C/T (71.4%), CZA (100%), and AK (100%), whereas they showed poor susceptibility to TZP (28.6%) and CAZ (28.6%). Among AmpC-p
K. aerogenes isolates, susceptibility to FEP and C/T were 100% (
n=4) and 50% (
n=2), respectively, whereas they were resistant to TZP and CAZ. The only AmpC/ESBL-p isolate showed resistance to TZP, CAZ and FEP, being susceptible to C/T. All the AmpC/ESBL-np isolates were susceptible to the antibiotics tested. All the
Klebsiella oxytoca,
Proteus mirabilis,
Citrobacter spp,
Pantoea agglomerans,
Morganella morganii, and
Salmonella spp isolates were CTX-M-CA-np, and they were susceptible to all the antibiotics tested. All the
Serratia marcenscens isolates were CTX-M-CA-np, and they showed full susceptibility to FEP, C/T, and CZA. Susceptibility to TZP, CAZ, and AK was 50% since one of the isolates was phenotypically characterized as AmpC-p showing resistance to these antibiotics. The only
Hafnia alvei isolate was classified as CTX-M-CA-np, being phenotypically characterized as AmpC-p, showing full susceptibility to TZP, FEP, CZA, AK, and resistance to CAZ, and C/T.
Table 2
Phenotypic characterization of Enterobacterales species included in the study and likelihood of deduction of in vitro antibiotic susceptibility according to conventional culture-based results
E. coli 68.2(161) | 20.5 (33) | - | - | - | - | 84.8 (28) | 30.3 (10) | 9.1 (3) | 97 (32) | 100 (33) | 75.8 (25) |
- | - | 0.6 (1) | - | 78.9 (127) | 90.6 (116) | 99.2 (127) | 100 (128) | 100 (128) | 100 (128) | 100 (128) |
K. pneumoniae 18.2(43) | 23.3 (10) | - | - | - | - | 40 (4) | 0 | 0 | 90 (9) | 100 (10) | 90 (9) |
- | 20.9 (9) | - | - | - | 0 | 0 | 0 | 0 | 88.9 (8) | 44.4 (4) |
- | - | 7 (3) | - | 48.8 (21) | 75 (18) | 87.5 (21) | 100 (24) | 95.8 (23) | 100 (24) | 100 (24) |
Other Enterobacterales 13.6(32) |
E. cloacae 4.2(10) | - | - | - | 20 (2) | 80 (8) | 80 (8) | 80 (8) | 80 (8) | 80 (8) | 100 (10) | 100 (10) |
K. aerogenes 3(7) | - | - | 57.1 (4) | 14.3 (1) | 28.6 (2) | 28.6 (2) | 28.6 (2) | 85.7 (6) | 71.4 (5) | 100 (7) | 100 (7) |
K. oxytoca 1.3(3) | - | - | - | - | 100 (3) | 100(3) | 100 (3) | 100 (3) | 100 (3) | 100 (3) | 100 (3) |
P. mirabilis 1.3(3) | - | - | - | - | 100 (3) | 100 (3) | 100 (3) | 100 (3) | 100 (3) | 100 (3) | 100 (3) |
S. marcenscens 0.9(2) | - | - | 50 (1) | - | 50 (1) | 50 (1) | 50 (1) | 100 (2) | 100 (2) | 100 (2) | 50 (1) |
Citrobacter spp 0.9(2) | - | - | - | - | 100 (2) | 100 (2) | 100 (2) | 100 (2) | 100 (2) | 100 (2) | 100 (2) |
P. agglomerans 0.9(2) | - | - | - | - | 100 (2) | 100 (2) | 100 (2) | 100 (2) | 100 (2) | 100 (2) | 100 (2) |
M. morganii 0.4(1) | - | - | - | - | 100 (1) | 100 (1) | 100 (1) | 100 (1) | 100 (1) | 100 (1) | 100 (1) |
Salmonella spp 0.4(1) | - | - | - | - | 100 (1) | 100 (1) | 100 (1) | 100 (1) | 100 (1) | 100 (1) | 100 (1) |
H. alvei 0.4(1) | - | - | 100 (1) | - | - | 100 (1) | 0 | 100 (1) | 0 | 100 (1) | 100 (1) |