In Vitro Activity of Ceftaroline against a Broad Spectrum of Recent Clinical Anaerobic Isolates

Abstract

The in vitro activity of ceftaroline was compared with those of ceftriaxone, clindamycin, imipenem, metronidazole, moxifloxacin, tigecycline, and vancomycin against 514 clinical anaerobic isolates using Clinical and Laboratory Standards Institute (CLSI) standard methodology. Ceftaroline demonstrated good to excellent activity against Gram-positive anaerobic pathogens and limited activity against Gram-negative pathogens, particularly Bacteroides fragilis group isolates.

Anaerobic bacteria are commonly present with aerobes in infections such as diabetic foot, aspiration pneumonia, and complicated intra-abdominal infections (3). In view of the increasing antibiotic resistance observed among anaerobes and their frequency of isolation, new agents effective against these pathogens would have potential utility for use in treating mixed aerobic/anaerobic infections (10, 11).

Ceftaroline is the active metabolite of ceftaroline fosamil, its N-phosphonoamino water-soluble prodrug. The agent has documented in vitro activity against methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant coagulase-negative staphylococci, drug-resistant Streptococcus pneumoniae (DRSP), and β-lactam-resistant Haemophilus influenzae and Moraxella catarrhalis isolates. In addition, ceftaroline exhibits in vitro activity against Gram-negative bacilli, with the exception of extended-spectrum β-lactamase (ESBL) producers, Pseudomonas and Acinetobacter species, as well as some Gram-positive anaerobic bacteria (1, 4, 6, 7, 8, 11).

To date, only limited studies evaluating the in vitro antianaerobe activity of ceftaroline have been published (1, 7, 11 [the study by Citron et al. describing the activity of ceftaroline against anaerobes was published while the manuscript was in preparation]). We conducted a comprehensive study to define the in vitro spectrum of activity of ceftaroline against a broad spectrum of recent clinical anaerobic bacteria.

(This study was presented in part at the 49th Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, 2009 [11].)

A total of 514 clinical anaerobic isolates collected from 2008 to 2009 were tested; 127 were Gram positive and 387 Gram negative. Some of the isolates were referred by the clinical laboratory at Tufts Medical Center. The Bacteroides fragilis group isolates belong to a collection referred by medical centers in the United States (9). Strains with elevated MICs to vancomycin and/or resistance to other antibiotics were included.

The following species were included among the 127 Gram-positive anaerobic clinical isolates from patients at Tufts Medical Center: Clostridium difficile (isolated from toxin-positive stools, with toxin typing not yet performed), other Clostridium spp., Peptostreptococcus spp., Propionibacterium spp., and other non-spore-forming Gram-positive bacilli. The Gram-negative isolates included species within the Bacteroides fragilis group (all referrals to the National Survey on Susceptibilities of the B. fragilis Group [9]), Prevotella spp., and a few Fusobacterium spp. To evaluate the potency of the new antibiotic, multidrug-resistant strains were included in the study. Identification of the isolates was confirmed by rapid methodologies, following the manufacturers' instructions (API 20A [bioMérieux, Inc., Durham, NC] and/or Rapid ID-ANA II [Innovative Diagnostic Systems, Inc., Norcross, GA]). When identification with the rapid methodology was not conclusive, the methods outlined in the Wadsworth-KTL Anaerobic Bacteriology Manual were used (5).

The in vitro activity of ceftaroline against Gram-positive anaerobic bacteria and Fusobacterium spp. was compared with those of ceftriaxone, clindamycin, imipenem, moxifloxacin, tigecycline, metronidazole, and vancomycin. The same agents, with the exception of vancomycin, were used to test isolates of the B. fragilis group and Prevotella spp.

Standard powder for ceftaroline (lot number FMD-CMF-019) was provided by Forest Laboratories, Inc., New York, NY. Standard powder for imipenem was provided by Merck and Company, West Point, PA, for moxifloxacin by Bayer Pharmaceuticals, West Haven, CT, and for tigecycline by Wyeth Ayerst Research, Pearl River, NY. Ceftriaxone, clindamycin, metronidazole, and vancomycin standard powders were purchased from the Sigma Corporation, St. Louis, MO. The manufacturers' instructions were followed to solubilize the compounds. Stock solutions, 10 times the desired highest test concentration, were prepared and kept frozen at −80°C until the day of use.

The MICs of the antibiotics were determined using agar dilution following CLSI recommendations and as previously described by Snydman et al. (2, 9, 10). American Type Culture Collection (ATCC) reference organisms were included with each test; Eubacterium lentum ATCC 43055 and C. difficile ATCC 00057 were used as controls in the testing of the Gram-positive isolates and B. fragilis ATCC 25285 and Bacteroides thetaiotaomicron ATCC 29741 were used in the testing of the Gram-negative isolates. Resistance rates for most of the antimicrobial agents were determined using accepted CLSI breakpoints for anaerobes (2). CLSI has not issued breakpoints for resistance for vancomycin, and thus, resistance breakpoints for this agent are not listed. For tigecycline, the U.S. Food and Drug Administration (FDA) recommended breakpoint for resistance of 16 μg/ml was used (12). Microsoft Excel (2003) was used for data storage and analysis.

The in vitro activities of ceftaroline compared with those of the other antimicrobial agents are listed in Tables 1 and 2. As indicated in Table 1, ceftaroline exhibited excellent activity against Peptostreptococcus spp. (MIC₉₀, 0.5 μg/ml), similar to the activities of vancomycin and tigecycline and superior to those of ceftriaxone, metronidazole, moxifloxacin, and clindamycin. Ceftaroline also showed excellent activity against Propionibacterium spp. (MIC₉₀ ≤ 0.06 μg/ml). Its activity against Propionibacterium spp. was superior to those of tigecycline and/or clindamycin.

TABLE 1.

Activities of ceftaroline and comparative agents against 127 Gram-positive anaerobic clinical isolates

Species (no. of isolates)	Antibiotic	MIC range (μg/ml)	MIC50 (μg/ml)	MIC90 (μg/ml)	% of isolates with resistancea
Clostridium difficile (26)
All isolates	Ceftaroline	≤0.015-8	2	8	NA
	Ceftriaxone	≤0.015->64	32	64	15
	Moxifloxacin	0.12-32	2	16	50
	Tigecycline	0.12-1	0.25	0.5	0
	Imipenem	≤0.06-8	4	8	0
	Clindamycin	≤0.06-16	2	16	31
	Metronidazole	≤0.12->32	16	32	46
	Vancomycin	≤0.12->32	2	4	NA
Quinolone-resistantb isolates (13)	Ceftaroline	0.5-8	2	4	NA
	Ceftriaxone	8->64	32	64	23
	Moxifloxacin	8-32	16	16	100
	Tigecycline	0.12-0.25	0.25	0.25	0
	Imipenem	2-8	4	8	0
	Clindamycin	0.5-16	2	16	38
	Metronidazole	1-32	32	32	54
	Vancomycin	1-4	2	2	NA
Quinolone-susceptibleb isolates (13)	Ceftaroline	≤0.015-8	2	8	NA
	Ceftriaxone	≤0.015-64	16	32	8
	Moxifloxacin	0.12-2	1	2	0
	Tigecycline	0.12-1	0.25	0.5	0
	Imipenem	≤0.06-4	2	4	0
	Clindamycin	≤0.06-16	1	16	23
	Metronidazole	≤0.5->32	16	>32	38
	Vancomycin	≤0.12->32	2	4	NA
C. perfringensc (7)	Ceftaroline	≤0.015-0.12			NA
	Ceftriaxone	≤0.015-0.12			0
	Moxifloxacin	0.25-1			0
	Tigecycline	0.25-4			0
	Imipenem	≤0.06-0.25			0
	Clindamycin	0.5-16			14
	Metronidazole	≤0.5-8			0
	Vancomycin	All 1			NA
Clostridium spp.d (16)	Ceftaroline	0.06-64	1	4	NA
	Ceftriaxone	0.125->64	8	32	6
	Moxifloxacin	0.05-32	1	16	31
	Tigecycline	≤0.06-0.5	0.125	0.25	0
	Imipenem	≤0.06-4	1	2	0
	Clindamycin	≤0.06-16	0.5	16	25
	Metronidazole	≤0.5->32	1	32	19
	Vancomycin	0.5-32	2	16	NA
Peptostreptococus spp.e (39)	Ceftaroline	≤0.015-8	0.03	0.5	NA
	Ceftriaxone	≤0.015->64	0.5	4	3
	Moxifloxacin	0.125-32	0.25	16	13
	Tigecycline	≤0.06-0.5	0.12	0.25	0
	Imipenem	≤0.06-0.5	0.06	0.125	0
	Clindamycin	≤0.06-16	0.06	1	8
	Metronidazole	≤0.5->32	1	>32	26
	Vancomycin	≤0.12-4	0.5	2	NA
Propionibacterium acnes (15)	Ceftaroline	≤0.015-0.06	0.03	0.06	NA
	Ceftriaxone	0.03-0.5	0.03	0.06	0
	Moxifloxacin	0.12-0.25	0.25	0.25	0
	Tigecycline	≤0.06-0.5	0.12	0.25	0
	Imipenem	≤0.06-≤0.06	≤0.06	≤0.06	0
	Clindamycin	≤0.06-0.12	≤0.06	0.125	0
	Metronidazole	≤0.5->32	32	>32	100
	Vancomycin	0.5-2	0.5	1	NA
Propionibacterium spp. (13)	Ceftaroline	≤0.015-0.06	0.03	0.03	0
	Ceftriaxone	≤0.015-0.12	0.03	0.06	0
	Moxifloxacin	0.12-2	0.25	0.5	0
	Tigecycline	≤0.06-0.25	≤0.06	0.125	0
	Imipenem	≤0.06-≤0.06	≤0.06	≤0.06	0
	Clindamycin	≤0.06-0.125	≤0.06	0.12	0
	Metronidazole	4->32	>32	>32	92
	Vancomycin	0.5-2	0.5	1	NA
Other Gram-positive non-spore-forming	Ceftaroline	≤0.015-1	0.12	0.5	NA
bacillif (11)	Ceftriaxone	≤0.015-64	0.5	32	9
	Moxifloxacin	0.12-2	2	2	0
	Tigecycline	0.12-1	0.5	0.5	0
	Imipenem	0.12-2	0.12	2	0
	Clindamycin	≤0.06-0.25	0.125	0.25	0
	Metronidazole	≤0.5-32	1	>32	55
	Vancomycin	≤0.12->32	32	>32	NA

^aThe breakpoint for resistance for most agents is the CLSI recommendation (2). For tigecycline, the FDA recommendation of 16 μg/ml was used (12). NA, not applicable, because CLSI has not established a breakpoint for resistance for vancomycin against anaerobes nor for resistance for ceftaroline.

^bQuinolone resistance was determined using moxifloxacin.

^cFor bacterial groups (species) with n < 10, the MIC range and percent resistance are the only statistics listed.

^dIncludes the following species: 3 C. cadaveris, 2 C. histolytica, 2 C. innocuum, 2 C. ramosum, 2 C. tertium, and 5 Clostridium spp.

^eIncludes the following species: 4 Finegoldia magna (previously P. Magnus), 2 Peptoniphilus asaccharolyticus (previously Peptostreptococcus asaccharolyticus), 1 P. tetradius, and 1 P. anaerobius; all others are Peptostreptococcus spp.

^fIncludes 3 Bifidobacterium spp., 6 Actinomyces spp., and 2 Lactobacillus spp.

TABLE 2.

Activities of ceftaroline and comparative agents against 387 Gram-negative anaerobic clinical isolates

Species (no. of isolates)	Antibiotic	MIC range (μg/ml)	MIC50 (μg/ml)	MIC90 (μg/ml)	% of isolates with resistancea
Bacteroides fragilis (205)	Ceftaroline	0.5->64	8	64	NA
	Ceftriaxone	0.12->64	32	>64	43
	Moxifloxacin	0.5->64	2	32	33
	Tigecycline	0.25-64	1	4	4.4
	Imipenem	0.12-32	0.25	0.5	1.5
	Clindamycin	≤0.5->64	≤0.5	>64	22
	Metronidazole	1-8	1	4	0
Bacteroides distasonis (22)	Ceftaroline	1->64	16	64	NA
	Ceftriaxone	0.25->64	>64	>64	55
	Moxifloxacin	2-64	4	64	41
	Tigecycline	0.25-4	2	4	0
	Imipenem	0.25-2	0.5	2	0
	Clindamycin	≤0.5->64	1	16	27
	Metronidazole	1-8	1	4	0
Bacteroides ovatus (36)	Ceftaroline	8-64	16	64	NA
	Ceftriaxone	0.25->64	64	>64	78
	Moxifloxacin	2->64	8	>64	81
	Tigecycline	0.12-32	2	16	8
	Imipenem	0.12-1	0.25	0.5	0
	Clindamycin	≤0.5->64	1	>64	31
	Metronidazole	1-4	1	4	0
Bacteroides thetaiotaomicron (69)	Ceftaroline	2-64	32	64	NA
	Ceftriaxone	0.12->64	>64	>64	80
	Moxifloxacin	0.25->64	8	>64	64
	Tigecycline	0.12->64	1	8	4
	Imipenem	0.12-8	0.25	1	0
	Clindamycin	≤0.5-64	2	64	39
	Metronidazole	1-8	2	4	0
Bacteroides uniformis (11)	Ceftaroline	0.03-64	8	16	NA
	Ceftriaxone	0.25->64	32	>64	45
	Moxifloxacin	0.06->64	4	>64	45
	Tigecycline	0.12-8	0.25	8	0
	Imipenem	0.12-1	0.25	1	0
	Clindamycin	≤0.5-16	≤0.5	16	27
	Metronidazole	1-4	2	4	0
Bacteroides vulgatus (20)	Ceftaroline	1-64	64	64	NA
	Ceftriaxone	0.25->64	64	>64	65
	Moxifloxacin	1->64	32	>64	65
	Tigecycline	0.12-16	1	4	5
	Imipenem	0.12-4	0.25	1	0
	Clindamycin	≤0.5->64	1	>64	45
	Metronidazole	1-4	1	2	0
Other Bacteroides spp.b,c (7)	Ceftaroline	4-64			NA
	Ceftriaxone	16->64			57
	Moxifloxacin	4-64			86
	Tigecycline	0.25-2			0
	Imipenem	0.12-05			0
	Clindamycin	≤0.5-16			29
	Metronidazole	1-2			0
Prevotella spp.d (14)	Ceftaroline	≤0.015-32	1	8	NA
	Ceftriaxone	≤0.015-64	2	8	7
	Moxifloxacin	0.125-32	0.5	4	7
	Tigecycline	0.06-2	0.5	1	0
	Imipenem	All ≤0.06			0
	Clindamycin	≤0.06-16	≤0.06	4	7
	Metronidazole	≤0.5->32	1	>32	21
Fusobacterium spp.b (3)	Ceftaroline	All ≤0.015			NA
	Ceftriaxone	≤0.015-0.06
	Moxifloxacin	0.125-0.25
	Tigecycline	≤0.06-0.125
	Imipenem	≤0.06-0.125
	Clindamycin	≤0.06-0.125
	Metronidazole	All ≤0.5

^aThe breakpoint for resistance for most agents is the CLSI recommendation (2). For tigecycline, the FDA recommendation of 16 μg/ml was used (12). NA, not applicable, because to date, there is no recommendation for ceftaroline.

^bFor bacterial groups (species) with n < 10, the MIC range and percent resistance are the only statistics listed.

^cIncludes 3 B. caccae and 4 B. eggherthii spp.

^dIncludes 3 P. bivia, 3 P. buccae, 3 P. melaninogenica, 1 P. intermedia, 1 P. loescheii, 1 P. denticola, and 2 Prevotella spp.

Ceftaroline exhibited excellent activity against Clostridium difficile and moderate activity against other Clostridium spp. The MIC range for ceftaroline against 26 isolates of C. difficile, which included isolates resistant to moxifloxacin, metronidazole, and clindamycin, as well as six isolates exhibiting elevated MICs to vancomycin (≥4 μg/ml), was 0.015 to 8 μg/ml, and the MIC₉₀ was 8 μg/ml. (The high rates of metronidazole resistance in some of the C. difficile isolates have not been examined with typing. It is possible some of these isolates represent a clone.) Against other Clostridium spp. (including 7 Clostridium perfringens isolates), the MIC₉₀ was 4 μg/ml.

The activity of ceftaroline against Gram-negative anaerobic isolates was varied (Table 2). Potent activity was observed against 3 isolates of Fusobacterium spp. (MIC ≤ 0.015 μg/ml), intermediate activity against Prevotella spp. (MIC₉₀, 8 μg/ml), and poor activity against the species of the B. fragilis group (MIC₉₀, 64 μg/ml). As with many cephalosporins, the activity of ceftaroline against the B. fragilis group was poor, consistent with the known inactivation by the β-lactamases produced by these pathogens. The activity of ceftaroline against Gram-negative anaerobes was comparable to that of ceftriaxone.

The results of this evaluation indicate that ceftaroline exhibits excellent activity against most of the Gram-positive anaerobic bacteria and poor activity against the Gram-negative anaerobic isolates, particularly isolates of the Bacteroides fragilis group. These findings are consistent with those reported by Citron et al. and Sader et al. (1, 7). Where differences might be noted, they could be explained by the number of species tested within the groups. Our results, as well as those of these authors, indicate that ceftaroline has promise as a therapeutic agent for skin and soft tissue mixed infections (1, 7, 9, 10, 11)