Activities of the Glycylcycline Tigecycline (GAR-936) against 1,924 Recent European Clinical Bacterial Isolates

Abstract

The in vitro activities of tigecycline against 1,924 clinical isolates were examined. The new glycylcycline exhibited excellent activity against all gram-positive cocci (MICs at which 90% of the isolates tested were inhibited [MIC₉₀s], ≤1 μg/ml). In addition, it was also very potent against most members of the Enterobacteriaceae, with most MIC₉₀s being ≤2 μg/ml. Among the nonfermenters, Acinetobacter spp. and Stenotrophomonas maltophilia are included in the in vitro spectrum of tigecycline activity.

Although tetracyclines remain valuable therapeutic agents for a variety of infections, resistance to this class limits their use. A new generation of tetracyclines, the glycylcyclines, is being developed specifically to overcome the problem of resistance to other tetracyclines (4, 16). Tigecycline (GAR-936), the 9-t-butylglycylamido derivative of minocycline, appears to be both better tolerated and more active against tetracycline-resistant strains than are other glycylcyclines (14). The present study examined the in vitro activities of tigecycline and comparators against recent clinical isolates from various European countries.

In all, 1,924 clinical isolates were tested. The strains were sampled between 1998 and 2001 in 28 hospitals from 16 European countries and sent to the Eijkman-Winkler Institute at the University Hospital Utrecht. Only one isolate per patient was permitted. The isolates derived from bloodstream, respiratory tract, skin and soft tissue, and urinary tract infections. Identification of the strains was performed by a VITEK or API system (Biomerieux, s′Hertogenbosch, The Netherlands) and/or by standard laboratory procedures.

MICs were determined by the microdilution method according to NCCLS guidelines (12) by using cation-adjusted Mueller-Hinton broth. For the testing of Streptococcus spp., the broth was supplemented with 5% lysed horse blood; for the testing of Haemophilus influenzae, Haemophilus Test Medium was used. The inoculum was adjusted to 5 × 10⁵ CFU/ml, and plates were read after incubation for 20 to 24 h at 35°C in ambient air.

Microtiter plates containing freeze-dried serial dilutions of the antibiotics were prepared by Trek Diagnostic Systems (Westlake, Ohio).

The results of the susceptibility testing of tigecycline and the comparator agents are shown in Table 1 and presented in terms of MIC₅₀ (MIC at which 50% of the isolates tested were inhibited), MIC₉₀, and the range of MICs.

TABLE 1.

In vitro activities of tigecycline and comparators

Organism (no. of strains) and antimicrobial agent	MIC (μg/ml)
	Range	50%	90%
Staphylococcus aureus, methicillin     susceptible (121)
Tigecycline	0.06-0.5	0.12	0.25
Minocycline	0.03-8	0.12	0.25
Oxacillin	0.06-2	0.5	1
Ceftriaxone	1-8	2	4
Vancomycin	0.5-2	1	1
Quinupristin-dalfopristin	0.12-1	0.25	0.5
Linezolid	1-4	2	4
Gatifloxacin	0.03-8	0.06	0.06
Staphylococcus aureus, methicillin     resistant (51)
Tigecycline	0.12-1	0.25	0.25
Minocycline	0.06-16	4	8
Oxacillin	4->16	>16	>16
Ceftriaxone	4->32	>32	>32
Vancomycin	0.5-2	1	1
Quinupristin-dalfopristin	0.25-1	0.5	1
Linezolid	1-4	2	4
Gatifloxacin	0.06-8	2	4
Staphylococcus epidermidis (85)
Tigecycline	0.06-1	0.25	0.5
Minocycline	0.06-2	0.25	0.5
Oxacillin	0.06->16	4	>16
Ceftriaxone	0.12->32	4	32
Vancomycin	0.5-2	1	2
Quinupristin-dalfopristin	0.12-2	0.25	0.25
Linezolid	0.25-4	1	1
Gatifloxacin	0.06-4	0.12	2
Staphylococcus haemolyticus (45)
Tigecycline	0.06-2	0.5	1
Minocycline	0.03-4	0.25	1
Oxacillin	0.5->16	>16	>16
Ceftriaxone	4->32	>32	>32
Vancomycin	0.5-4	1	2
Quinupristin-dalfopristin	0.12-2	0.25	0.5
Linezolid	0.5-2	1	2
Gatifloxacin	0.06-16	4	8
Enterococcus faecalis (124)
Tigecycline	≤0.015-1	0.12	0.25
Minocycline	0.03->32	8	16
Oxacillin	8->16	>16	>16
Ceftriaxone	8->32	>32	>32
Vancomycin	0.5->128	1	2
Quinupristin-dalfopristin	0.25->32	8	32
Linezolid	1-4	2	2
Gatifloxacin	0.12->16	0.5	16
Enterococcus faecium (114)
Tigecycline	0.03-0.25	0.06	0.12
Minocycline	0.06-32	16	16
Oxacillin	4->16	>16	>16
Ceftriaxone	4->32	>32	>32
Vancomycin	0.25->128	1	>128
Quinupristin-dalfopristin	0.25-32	1	4
Linezolid	1-4	2	2
Gatifloxacin	0.12->16	4	>16
Group A streptococci (135)
Tigecycline	≤0.015-0.5	0.6	0.6
Minocycline	0.03-16	0.06	8
Oxacillin	≤0.03-0.25	0.06	0.06
Ceftriaxone	≤0.015-0.06	≤0.015	0.03
Vancomycin	0.25-1	0.5	0.5
Quinupristin-dalfopristin	0.12-1	0.12	0.25
Linezolid	0.5-2	1	1
Gatifloxacin	0.12-2	0.25	0.5
Group B streptococci (23)
Tigecycline	0.03-0.06	0.06	0.06
Minocycline	0.06-16	16	16
Oxacillin	0.25-0.5	0.5	0.5
Ceftriaxone	0.06-0.12	0.06	0.12
Vancomycin	0.5-0.5	0.5	0.5
Quinupristin-dalfopristin	0.25-1	0.25	0.5
Linezolid	1-2	1	2
Gatifloxacin	0.12-0.5	0.25	0.5
Streptococcus pneumoniae,     penicillin susceptible (107)
Tigecycline	≤0.015-1	0.03	0.25
Minocycline	0.03-16	0.12	0.5
Oxacillin	≤0.03-2	≤0.03	0.25
Ceftriaxone	≤0.015-0.25	≤0.015	0.03
Vancomycin	0.12-0.5	0.5	0.5
Quinupristin-dalfopristin	0.25-1	0.5	1
Linezolid	0.25-1	1	1
Gatifloxacin	0.06-0.5	0.25	0.5
Streptococcus pneumoniae,     penicillin intermediate (26)
Tigecycline	0.03-1	0.06	0.5
Minocycline	0.06-32	0.5	16
Oxacillin	≤0.03-16	2	16
Ceftriaxone	≤0.015-1	0.12	1
Vancomycin	0.06-0.5	0.25	0.5
Quinupristin-dalfopristin	0.25-1	0.5	1
Linezolid	0.25-1	1	1
Gatifloxacin	0.12-0.5	0.25	0.25
Streptococcus pneumoniae,     penicillin resistant (17)
Tigecycline	0.03-1	0.12	0.5
Minocycline	0.06-16	0.25	16
Oxacillin	0.5->16	16	>16
Ceftriaxone	0.25-2	1	1
Vancomycin	0.25-0.5	0.25	0.5
Quinupristin-dalfopristin	0.25-1	0.5	1
Linezolid	0.25-1	1	1
Gatifloxacin	0.12-1	0.25	0.5
Viridans group streptococci (86)
Tigecycline	≤0.015-2	0.12	0.5
Minocycline	0.06-32	0.5	16
Oxacillin	≤0.03->16	0.25	2
Ceftriaxone	≤0.015-8	0.12	1
Vancomycin	≤0.06-1	0.5	1
Quinupristin-dalfopristin	0.12-2	1	1
Linezolid	0.5-2	1	1
Gatifloxacin	0.12-2	0.25	0.5
Escherichia coli (100)
Tigecycline	0.06-1	0.25	0.5
Minocycline	0.25-64	1	8
Piperacillin	0.5->128	128	>128
Piperacillin-tazobactam	0.25->128	2	4
Ceftriaxone	≤0.06-4	≤0.06	≤0.06
Imipenem	≤0.06-2	0.12	0.25
Gatifloxacin	≤0.008-16	0.03	0.5
Tobramycin	0.25-8	1	2
Klebsiella pneumoniae (101)
Tigecycline	0.25-4	0.5	1
Minocycline	1->64	2	32
Piperacillin	2->128	16	>128
Piperacillin-tazobactam	0.5->128	4	16
Ceftriaxone	≤0.06->128	≤0.06	32
Imipenem	0.12-2	0.25	0.5
Gatifloxacin	0.03-16	0.06	1
Tobramycin	0.12->32	0.5	16
Klebsiella oxytoca (49)
Tigecycline	0.5-2	0.5	1
Minocycline	1->64	2	8
Piperacillin	2->128	16	>128
Piperacillin-tazobactam	1->128	2	>128
Ceftriaxone	≤0.06-128	0.12	32
Imipenem	0.12-0.5	0.25	0.5
Gatifloxacin	0.03-4	0.06	0.5
Tobramycin	0.25->32	0.5	>32
Proteus mirabilis (88)
Tigecycline	1-8	4	8
Minocycline	4->64	64	>64
Piperacillin	0.12->128	0.5	>128
Piperacillin-tazobactam	≤0.06-128	0.5	1
Ceftriaxone	≤0.06-64	≤0.06	≤0.06
Imipenem	0.5-8	4	8
Gatifloxacin	0.12->16	0.25	8
Tobramycin	0.25->32	2	8
Proteus vulgaris (30)
Tigecycline	0.12-16	4	4
Minocycline	0.5->64	8	64
Piperacillin	0.25->128	1	>128
Piperacillin-tazobactam	0.12-2	0.25	1
Ceftriaxone	≤0.06-64	≤0.06	4
Imipenem	0.25-4	4	4
Gatifloxacin	0.03-16	0.12	0.25
Tobramycin	0.5-4	1	2
Serratia marcescens (61)
Tigecycline	0.5-4	2	4
Minocycline	2-32	4	16
Piperacillin	1->128	4	>128
Piperacillin-tazobactam	0.5->128	2	32
Ceftriaxone	≤0.06->128	0.25	16
Imipenem	0.5-8	2	2
Gatifloxacin	0.06->16	0.25	4
Tobramycin	0.5->32	2	16
Enterobacter cloacae (81)
Tigecycline	0.25-4	1	2
Minocycline	2-32	4	8
Piperacillin	0.5->128	4	>128
Piperacillin-tazobactam	1->128	2	128
Ceftriaxone	≤0.06->128	0.25	>128
Imipenem	0.12-2	0.5	1
Gatifloxacin	0.015-16	0.06	0.25
Tobramycin	0.25-32	0.5	1
Enterobacter aerogenes (42)
Tigecycline	0.25-8	1	2
Minocycline	1-64	4	16
Piperacillin	2->128	128	>128
Piperacillin-tazobactam	1->128	16	128
Ceftriaxone	≤0.06->128	8	64
Imipenem	0.25-2	1	2
Gatifloxacin	0.03->16	0.12	>16
Tobramycin	0.25-32	1	16
Citrobacter freundii (41)
Tigecycline	0.25-16	0.5	2
Minocycline	1->64	4	32
Piperacillin	1->128	4	>128
Piperacillin-tazobactam	1->128	4	64
Ceftriaxone	≤0.06->128	0.5	128
Imipenem	0.25-4	1	2
Gatifloxacin	0.015->16	0.06	2
Tobramycin	0.25-32	0.5	16
Morganella morganii (34)
Tigecycline	1-8	2	4
Minocycline	2->64	8	64
Piperacillin	0.25->128	1	64
Piperacillin-tazobactam	0.12->128	0.25	0.5
Ceftriaxone	≤0.06-128	≤0.06	0.5
Imipenem	2-4	4	4
Gatifloxacin	0.03-16	0.12	2
Tobramycin	0.25-4	0.5	2
Pseudomonas aeruginosa (50)
Tigecycline	0.5-32	16	32
Minocycline	2->64	16	32
Piperacillin	2->128	16	>128
Piperacillin-tazobactam	0.25->128	8	>128
Ceftriaxone	8->128	128	>128
Imipenem	0.25->128	2	32
Gatifloxacin	0.25->16	1	>16
Tobramycin	0.25->32	1	>32
Acinetobacter spp. (50)
Tigecycline	0.06-4	0.25	2
Minocycline	≤0.03-16	0.12	8
Piperacillin	1->128	32	>128
Piperacillin-tazobactam	≤0.06->128	16	>128
Ceftriaxone	0.12->128	16	>128
Imipenem	≤0.06->128	0.5	32
Gatifloxacin	0.03-16	0.12	16
Tobramycin	0.12->32	1	32
Stenotrophomonas maltophilia     (39)
Tigecycline	0.25-4	0.5	2
Minocycline	0.12-1	0.25	0.5
Piperacillin	8->128	>128	>128
Piperacillin-tazobactam	8->128	>128	>128
Ceftriaxone	32->128	>128	>128
Imipenem	32->128	>128	>128
Gatifloxacin	0.12-4	0.5	2
Tobramycin	1->32	32	>32
Burkholderia cepacia (20)
Tigecycline	0.5-64	4	32
Minocycline	1-64	2	16
Piperacillin	2->128	8	128
Piperacillin-tazobactam	1->128	4	64
Ceftriaxone	0.5->128	4	>128
Imipenem	0.25-32	8	32
Gatifloxacin	0.25->16	2	>16
Tobramycin	8->32	>32	>32
Haemophilus influenzae (103)
Tigecycline	0.12-2	0.5	1
Minocycline	0.12-2	0.5	1
Piperacillin	≤0.06-64	≤0.06	0.25
Piperacillin-tazobactam	≤0.06-0.5	≤0.06	≤0.06
Ceftriaxone	≤0.06-0.12	≤0.06	≤0.06
Imipenem	≤0.06-2	0.5	1
Gatifloxacin	≤0.008-0.06	≤0.008	0.015
Tobramycin	0.5-8	2	4
Moraxella catarrhalis (101)
Tigecycline	≤0.03-0.25	0.06	0.12
Minocycline	≤0.03-1	0.06	0.12
Piperacillin	≤0.06-16	0.5	4
Piperacillin-tazobactam	≤0.06-≤0.06	≤0.06	≤0.06
Ceftriaxone	≤0.06-1	≤0.06	0.5
Imipenem	≤0.06-0.5	≤0.06	0.12
Gatifloxacin	≤0.008-0.25	0.03	0.03
Tobramycin	≤0.06-0.25	0.12	0.25

Tigecycline was very active against staphylococci (range of MIC₅₀s, 0.12 to 0.5 μg/ml; range of MIC₉₀s, 0.25 to 1 μg/ml), with no difference being observed between the MIC₉₀s for methicillin-susceptible and methicillin-resistant strains. Tigecycline was the most active agent against methicillin-resistant Staphylococcus aureus strains (MIC₅₀ and MIC₉₀, 0.25 μg/ml). Other authors have described an in vitro activity of minocycline against staphylococci that is higher than that of tigecycline (3, 6); however, in our study the new glycylcycline had a higher activity, with a concentration of 1 μg/ml inhibiting all S. aureus strains, including minocycline-resistant isolates, and a concentration of 2 μg/ml inhibiting all coagulase-negative staphylococci. A possible explanation for the lower minocycline activity in our study could be that in our bacterial population, the proportion of tetracycline-resistant isolates harboring the tet M gene was greater. As has been shown previously, 50% of the tetracycline-resistant Staphylococcus aureus strains in Europe carry the tet M gene, which confers resistance to both tetracycline and minocycline (15). In contrast, isolates harboring only the tet K gene are resistant to tetracycline but remain susceptible to minocycline.

Against Enterococcus faecalis and Enterococcus faecium, tigecycline showed high activity, with MIC₅₀s of 0.12 and 0.06 μg/ml, respectively, and MIC₉₀s of 0.25 and 0.12 μg/ml, respectively. It was the most active agent among all antibiotics tested. Approximately half of the enterococcal isolates were resistant to minocycline. However, all strains, including 27 vancomycin-resistant strains, were inhibited by tigecycline at a concentration of 1 μg/ml. Based on the MIC₉₀s, tigecycline was at least eightfold more active than linezolid against Enterococcus spp. and 32-fold more active than quinupristin-dalfopristin against Enterococcus faecium.

Tigecycline was also highly active against beta-hemolytic streptococci belonging to Lancefield groups A and B (MIC₅₀ and MIC₉₀, 0.06 μg/ml), with all strains, including minocycline-resistant isolates, being inhibited by the antibiotic at a concentration of 0.5 μg/ml. The difference in activities between tigecycline and minocycline was most pronounced with group B streptococci: 78% of the strains were resistant to minocycline, with all of them being inhibited by 0.06 μg of tigecycline/ml.

Against pneumococci and viridans group streptococci, the MIC₅₀s of tigecycline were 0.06 and 0.12 μg/ml, respectively, and the MIC₉₀s were 0.5 μg/ml for both. For penicillin-susceptible and nonsusceptible pneumococcal strains, the MIC₉₀s differed by only 1 dilution step (0.25 and 0.5 μg/ml, respectively). All pneumococcal isolates, including those resistant to minocycline (12.7%), were inhibited by tigecycline at a concentration of 1 μg/ml. Based on the MIC₉₀s, the potency of tigecycline was found to be equivalent to that of gatifloxacin and slightly superior to that of linezolid and quinupristin-dalfopristin.

Overall, the new glycylcycline exhibited very high activity against gram-positive bacteria, including multiresistant pathogens and minocycline-resistant isolates, all of which were inhibited by tigecycline at a concentration of 1 μg/ml. These results confirm and extend those of previous analyses using gram-positive clinical isolates mainly from the United States (1-3, 6, 7, 13, 14, 17).

Tigecycline exhibited very high activity against Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, Enterobacter cloacae, Enterobacter aerogenes, and Citrobacter freundii, with MIC₅₀s ranging from 0.25 to 1 μg/ml and MIC₉₀s ranging from 0.5 to 2 μg/ml. Against these species, tigecycline was four- to eightfold more active than minocycline based on a comparison of the MIC₅₀s. Against Proteus mirabilis, Proteus vulgaris, Morganella morganii, and Serratia marcescens, tigecycline was less active, with MIC₅₀s of 4, 4, 2, and 2 μg/ml, respectively, and MIC₉₀s of 8, 4, 4, and 4 μg/ml, respectively. Compared with minocycline, it was 2- to 16-fold more active based on the MIC₅₀s. Overall, 172 (27.4%) of the 627 enterobacterial strains tested were resistant to minocycline (MIC, ≥16 μg/ml). Interestingly, 65 (37.8%) of the 172 minocycline-resistant strains were inhibited by tigecycline at a concentration of 2 μg/ml. These results extend those of previous analyses using Enterobacteriaceae isolates mainly from the United States (1, 2, 6-8, 14, 17).

Remarkably, tigecycline showed high activity against defined gram-negative nonfermentative bacteria, i.e., Acinetobacter spp. (MIC₅₀, 0.25 μg/ml; MIC₉₀, 2 μg/ml) and Stenotrophomonas maltophilia (MIC₅₀, 0.5 μg/ml; MIC₉₀, 2 μg/ml), two multiresistant bacterial species often involved in nosocomial infections. Based on the MIC₅₀s, tigecycline was 1 dilution less active than minocycline. However, isolates of Acinetobacter that were not susceptible to minocycline were all inhibited by tigecycline at a concentration of 2 μg/ml. Minocycline and tigecycline both exhibited lower potency against Burkholderia cepacia, with minocycline being slightly more active than tigecycline (MIC₅₀s of 2 and 4 μg/ml, respectively, and MIC₉₀s of 16 and 32 μg/ml, respectively). Against Pseudomonas aeruginosa, neither minocycline nor tigecycline had any clinically relevant activity. These results are in accordance with those of previous limited analyses using nonfermentative bacteria (1, 2, 6-8, 14, 17).

Against Haemophilus influenzae, the activity of tigecycline was comparable to that of minocycline and imipenem (MIC_50, 0.5 μg/ml; MIC₉₀, 1 μg/ml). However, gatifloxacin, piperacillin-tazobactam, and ceftriaxone were at least eightfold more active against this species (MIC₅₀s and MIC₉₀s, ≤0.06 μg/ml).

Against Moraxella catarrhalis, tigecycline and minocycline were equally active, with both agents showing an MIC₅₀ of 0.06 μg/ml and an MIC₉₀ of 0.12 μg/ml (1, 6, 9, 14, 17). In addition, it has been demonstrated that the glycylcyclines are not only active against the classical respiratory pathogens, i.e., Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis, but also are active against Mycoplasma pneumoniae and Chlamydia pneumoniae (5, 10).

The results of our in vitro investigation of tigecycline are in line with those of previous analyses that demonstrated its broad spectrum of antibacterial activity (1, 2, 3, 6, 11, 14, 17). Tigecycline exhibited excellent activity against gram-positive cocci (MIC₉₀s, ≤1 μg/ml), including methicillin-resistant staphylococci, penicillin-resistant pneumococci, and vancomycin-resistant enterococci. Tigecycline also showed very high potency against members of the Enterobacteriaceae, with MIC₉₀s of ≤2 μg/ml, with the exception of Proteus spp., Serratia marcescens, and Morganella morganii (MIC₉₀s of 4 to 8 μg/ml). In general, it was two- to eightfold more active than minocycline against the enteric bacilli. Among the nonfermenters, Acinetobacter spp. and Stenotrophomonas maltophilia are included in the in vitro spectrum of tigecycline activity. In addition, a shift to lower MICs compared with the MIC distributions of minocycline was observed with several gram-negative species, e.g., Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, and Acinetobacter spp. Tigecycline exhibited high activity not only against pneumococci but also against the other two major respiratory tract pathogens, Haemophilus influenzae and Moraxella catarrhalis.

These results indicate that tigecycline holds promise as an important therapeutic option for infections caused by gram-positive and gram-negative bacteria, especially in view of the increasing problem of the resistance of various species to other classes of antibiotics.