ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Sept. 2011, p. 3985–3989
Copyright © 2011, American Society for Microbiology. All Rights Reserved.
Vol. 55, No. 9
Antimicrobial Susceptibility to Azithromycin among Salmonella enterica
Isolates from the United States?
Maria Sjo ¨lund-Karlsson,1* Kevin Joyce,1Karen Blickenstaff,2Takiyah Ball,3Jovita Haro,3
Felicita M. Medalla,1Paula Fedorka-Cray,3Shaohua Zhao,2
John A. Crump,1and Jean M. Whichard1
Division of Foodborne, Waterborne, and Environmental Diseases, Centers for Disease Control and Prevention, Atlanta,
Georgia1; Food and Drug Administration-Center for Veterinary Medicine (FDA-CVM), Laurel, Maryland2; and
USDA-ARS Bacterial Epidemiology and Antimicrobial Resistance Research Unit, Athens, Georgia3
Received 28 April 2011/Returned for modification 5 June 2011/Accepted 9 June 2011
Due to emerging resistance to traditional antimicrobial agents, such as ampicillin, trimethoprim-sulfa-
methoxazole, and chloramphenicol, azithromycin is increasingly used for the treatment of invasive Salmonella
infections. In the present study, 696 isolates of non-Typhi Salmonella collected from humans, food animals, and
retail meats in the United States were investigated for antimicrobial susceptibility to azithromycin. Seventy-two
Salmonella enterica serotype Typhi isolates from humans were also tested. For each isolate, MICs of azithro-
mycin and 15 other antimicrobial agents were determined by broth microdilution. Among the non-Typhi
Salmonella isolates, azithromycin MICs among human isolates ranged from 1 to 32 ?g/ml, whereas the MICs
among the animal and retail meat isolates ranged from 2 to 16 ?g/ml and 4 to 16 ?g/ml, respectively. Among
Salmonella serotype Typhi isolates, the azithromycin MICs ranged from 4 to 16 ?g/ml. The highest MIC
observed in the present study was 32 ?g/ml, and it was detected in three human isolates belonging to serotypes
Kentucky, Montevideo, and Paratyphi A. Based on our findings, we propose an epidemiological cutoff value
(ECOFF) for wild-type Salmonella of <16 ?g/ml of azithromycin. The susceptibility data provided could be
used in combination with clinical outcome data to determine tentative clinical breakpoints for azithromycin
and Salmonella enterica.
Non-Typhi Salmonella is the second leading cause of food-
borne illness in the United States (34). Each year, approxi-
mately 1.0 million people are infected with Salmonella, result-
ing in 19,000 hospitalizations and almost 400 deaths (34).
Although antimicrobial treatment is not indicated for uncom-
plicated infections, antimicrobial agents are potentially life-
saving for extraintestinal infections. Likewise, antimicrobial
therapy is essential for treating infections of Salmonella en-
terica serotypes Typhi and Paratyphi, the causative agents of
enteric fever (31).
Due to widespread resistance to traditional first-line drugs,
such as ampicillin, trimethoprim-sulfamethoxazole, and chlor-
amphenicol, current recommendations suggest using a fluoro-
quinolone (e.g., ciprofloxacin) or an extended-spectrum ceph-
alosporin (e.g., ceftriaxone) for treating invasive and severe
Salmonella infections (16, 18). However, Salmonella serotype
Typhi and paratyphoidal serotypes with decreased susceptibil-
ities to fluoroquinolones, which may be associated with inad-
equate responses to treatment, have become common, and
reports of fluoroquinolone-resistant Salmonella strains are in-
creasing (8, 22, 24, 25). Furthermore, extended-spectrum
?-lactamase-producing Salmonella serotype Typhi and Salmo-
nella serotype Paratyphi A have been reported (1, 32). Of even
greater concern, some Salmonella isolates display concurrent
resistance to quinolones and extended-spectrum cephalospo-
rins, which may require use of an alternative antimicrobial
class for management of invasive infections (27, 36).
Azithromycin is an azalide antimicrobial agent that has been
demonstrated in clinical trials to be equivalent or superior to
chloramphenicol, fluoroquinolones, and extended-spectrum
cephalosporins for the management of uncomplicated typhoid
fever (3, 4, 13–15, 30). In addition, azithromycin is increasingly
considered for the management of bacillary dysentery and in-
vasive nontyphoidal Salmonella infections (9, 10, 37, 38). Azi-
thromycin shows excellent penetration into most tissues, and it
achieves concentrations in macrophages and neutrophils that
are ?100-fold higher than concentrations in serum (23, 29).
These properties, along with azithromycin’s long half-life of 2
to 3 days, have increased azithromycin’s role in the therapeutic
management of infections of the reticuloendothelial system.
Prior to initiation of any antimicrobial therapy, the in vitro
susceptibility of the disease-causing bacteria to potentially
effective antimicrobial agents should be examined. The in
vitro susceptibilities of bacteria are classified as either “sus-
ceptible,” “intermediate,” or “resistant” and are defined by
clinical breakpoints. In the United States, the Food and
Drug Administration (FDA) and the Clinical and Labora-
tory Standards Institute (CLSI) define breakpoints. In Eu-
rope, the European Committee on Antimicrobial Suscepti-
bility Testing (EUCAST) is responsible for defining clinical
breakpoints for new and existing drugs (20, 21).
At present, no clinical azithromycin breakpoints have been
defined for Enterobacteriaceae, including Salmonella, by either
the CLSI or EUCAST (5, 6, 11). By performing antimicrobial
* Corresponding author. Mailing address: National Antimicrobial
Resistance Monitoring System, Centers for Disease Control and Pre-
vention, CCID/NCZVED/DFBMD/EDLB, 1600 Clifton Road, At-
lanta, GA 30329. Phone: (404) 639-0698. Fax: (404) 639-4290. E-mail:
?Published ahead of print on 20 June 2011.
susceptibility testing on 696 isolates of non-Typhi Salmonella
isolated from humans, food animals, and retail meats in the
United States, we provide data on the range of azithromycin
MICs observed among Salmonella enterica isolates from the
United States. We further present azithromycin MICs for 72
isolates of Salmonella serotype Typhi. This information could
be combined with clinical outcome data to facilitate establish-
ment of clinical azithromycin breakpoints for Salmonella.
MATERIALS AND METHODS
In 2008, 54 state and local public health laboratories participating in the
National Antimicrobial Resistance Monitoring System (NARMS) forwarded
every isolate of Salmonella serotype Typhi and every 20th non-Typhi Salmonella
isolate from human clinical infections to the Centers for Disease Control and
Prevention (CDC). Similarly, non-Typhi Salmonella isolates from retail meats
(chicken breasts, ground turkey, ground beef, and pork chops) were submitted by
the states participating in the Foodborne Diseases Active Surveillance Network
(FoodNet) for analysis at the U.S. Food and Drug Administration Center for
Veterinary Medicine (FDA-CVM). Non-Typhi Salmonella isolates from food
animals were obtained from carcass rinsates (chicken), carcass swabs (turkey,
cattle, and swine), and ground products (chicken, turkey, and beef). Animal
samples were collected by the U.S. Department of Agriculture’s (USDA) Food
Safety Inspection Service (FSIS) from federally inspected slaughter and process-
ing plants throughout the United States and forwarded to the USDA in Athens,
GA, for further analysis. In addition, non-Typhi Salmonella isolates from animal
products were collected through special studies performed at the USDA. At each
agency, 232 non-Typhi Salmonella isolates were randomly chosen for azithromy-
cin susceptibility determination. Moreover, 60 isolates of Salmonella serotype
Typhi submitted in 2008 and 12 additional isolates from previous years were
tested for azithromycin susceptibility at the CDC. All Typhi isolates were clinical
isolates collected from humans.
MICs were determined by broth microdilution using a frozen azithromycin
reference panel with concentrations ranging from 0.25 to 256 ?g/ml (Sensititre;
Trek Diagnostics, Westlake, OH). Antimicrobial susceptibility testing was per-
formed according to the manufacturer’s instructions, with Staphylococcus aureus
ATCC 29213 used as a quality control strain (5). For each isolate, a final
inoculum of 5 ? 105CFU/ml was targeted. The panels were read after 18 h of
incubation at 35°C.
In addition, all isolates were tested for susceptibility to 15 antimicrobial agents
included on the NARMS Gram-negative panel (amikacin, ampicillin, amoxicil-
lin-clavulanic acid, ceftiofur, ceftriaxone, cefoxitin, chloramphenicol, ciprofloxa-
cin, gentamicin, kanamycin, nalidixic acid, sulfamethoxazole, streptomycin, tri-
methoprim-sulfamethoxazole, and tetracycline) and interpreted according to
CLSI standards, where available (5). Additional susceptibility testing with azi-
thromycin Etest (bioMe ´rieux, Inc., NC) strips was performed according to the
manufacturer’s instructions on Mueller-Hinton II agar plates incubated at 37°C
for 16 to 20 h.
An MIC histogram was constructed, and the MIC50value, representing the
MIC at which the growth of 50% of the population is inhibited, was calculated for
each of the four sample groups (Salmonella serotype Typhi from humans and
non-Typhi Salmonella from humans, retail meats, and animals). The histograms
were further inspected to identify the wild-type MIC distribution.
Genomic DNA was prepared by lysing the bacteria at 95°C and collecting the
supernatant following centrifugation. A PCR assay using previously described
primers was used to screen for the following macrolide resistance genes: ereA,
ereB, ermB, mefA, mphA, mphB, and mphD (28). Plasmids were extracted using
the Qiagen plasmid midi prep kit (Qiagen, Valencia, CA) and electroporated
into electrocompetent Escherichia coli DH10B (Invitrogen, Carlsbad, CA).
In 2008, 2,379 isolates of non-Typhi Salmonella were sub-
mitted to the CDC, 1,326 to the USDA, and 495 to the FDA-
CVM as part of the NARMS program for enteric bacteria. At
each agency, a subset of 232 isolates was randomly chosen to
be tested for antimicrobial susceptibility to azithromycin. In
addition, 72 isolates of Salmonella serotype Typhi were in-
cluded. Among the Salmonella serotype Typhi isolates, 49
(68.1%) were isolated from blood cultures and 16 (22.2%)
from stool. The sources of the remaining 7 isolates were not
provided by the submitting laboratory.
Among the 232 non-Typhi Salmonella isolates randomly se-
lected from human isolate submissions, Salmonella enterica
serotypes Enteritidis (19.0%) and Typhimurium (13.4%) were
most common. Of these, 84.1% were isolated from stool cul-
tures and 9.1% from blood cultures. The remaining 16 isolates
were isolated from urine, other sources, or nonspecified
sources. Among the isolates randomly selected for analysis
from food animal submissions, Salmonella enterica serotypes
Kentucky (16.4%), Heidelberg (9.5%), and Montevideo
(7.3%) were most common, and among the isolates randomly
selected from retail meats, Salmonella enterica serotypes
Heidelberg (19.6%), Hadar (16.8%), and Typhimurium vari-
ant O:5? (10.8%) predominated. Among the animal isolates,
227 were obtained from the following sample sources: chicken
(38.8%), cattle (37.9%), turkey (15.4%), and swine (7.9%).
The remaining five isolates from the USDA were special study
isolates obtained from ready-to-eat products and eggs. Among
the retail meat isolates, 116 (50.0%) originated from ground
turkey, 95 (40.9%) from chicken breast, 12 (5.2%) from pork
chops, and 9 (3.9%) from ground beef.
Among the non-Typhi Salmonella isolates from all sources,
64 (9.2%) displayed resistance to ceftriaxone (MIC ? 4 ?g/ml)
and 15 (2.2%) showed decreased susceptibility to ciprofloxacin
(MIC ? 0.125 ?g/ml). The azithromycin MICs among the
isolates collected from humans ranged from 1 to 32 ?g/ml,
whereas the MICs among the animal and retail meat isolates
ranged from 2 to 16 ?g/ml and 4 to 16 ?g/ml, respectively (Fig.
1). All three distributions peaked at 8 ?g/ml, and the MIC50
value for each distribution was 8 ?g/ml. Three isolates col-
lected from humans displayed an MIC of 32 ?g/ml. These
isolates comprised Salmonella serotypes Kentucky, Montevi-
deo, and Paratyphi A. The serotype Paratyphi A isolate was
obtained from blood, whereas the serotype Kentucky and
Montevideo isolates were obtained from stool. By PCR, these
three isolates were negative for genes associated with macro-
lide resistance (ereA, ereB, ermB, mefA, mphA, mphB, and
mphD) (28). Moreover, plasmid extractions and electropora-
tion of plasmids into E. coli DH10B did not yield any trans-
formants. Retesting of these isolates by Etest yielded MICs of
Among the 72 isolates of Salmonella serotype Typhi, 5
(7.0%) showed resistance to ampicillin (MIC ? 32 ?g/ml), 3
(4.2%) to chloramphenicol (MIC ? 32 ?g/ml), and 7 (9.7%) to
trimethoprim-sulfamethoxazole (MIC ? 4 ?g/ml). Fifty-eight
(81.0%) isolates showed decreased susceptibility to ciprofloxa-
cin (MIC ? 0.125 ?g/ml). Of these, 12 (16.7%) showed clinical
resistance (ciprofloxacin MIC ? 4 ?g/ml), according to current
CLSI guidelines (6). The azithromycin MICs among the Sal-
monella serotype Typhi isolates ranged from 4 to 16 ?g/ml
Currently, azithromycin is recommended for the treatment
of both shigellosis and invasive salmonellosis by the World
Health Organization and the American Academy of Pediatrics
(2, 37, 38) and is increasingly used for the management of
3986SJO ¨LUND-KARLSSON ET AL.ANTIMICROB. AGENTS CHEMOTHER.
uncomplicated enteric fever (3, 4, 13–15, 30). However, clinical
breakpoints for azithromycin and Salmonella have yet to be
defined. Clinical breakpoints are necessary to detect emerging
and changing patterns of resistance and to guide clinicians in
the selection of effective antimicrobial therapy. The first step
toward defining clinical breakpoints is to collect relevant data,
including (i) pharmacodynamic data of the drug, (ii) pharma-
cological properties of the drug, (iii) clinical outcome data, and
(iv) microbiological data, i.e., MIC data for the specific patho-
gen in question (7, 35). In this paper, we provide MIC data for
696 isolates of non-Typhi Salmonella and 72 isolates of Salmo-
nella serotype Typhi that could contribute to establishing sus-
ceptibility breakpoints for Salmonella and azithromycin.
The 696 non-Typhi Salmonella isolates included in the pres-
ent study were collected from various sources, including hu-
mans, animals, and retail meats in the United States. The MIC
distributions for the different sample sources were similar, and
the majority of the isolates in each distribution displayed MIC
FIG. 1. Azithromycin MIC distributions for 696 isolates of non-Typhi Salmonella enterica collected from humans, food animals and retail meats
through the National Antimicrobial Resistance Monitoring System (NARMS), 2008. The dashed line denotes a proposed epidemiological cutoff
value (ECOFF) for WT isolates of ?16 ?g/ml.
FIG. 2. Azithromycin MIC distribution for 72 isolates of S. enterica serotype Typhi collected by the National Antimicrobial Resistance
Monitoring System (NARMS). The dashed line denotes a proposed epidemiological cutoff value (ECOFF) for WT isolates of ?16 ?g/ml.
VOL. 55, 2011 AZITHROMYCIN SUSCEPTIBILITY IN S. ENTERICA 3987
values of 4 to 8 ?g/ml. Thus, overall, azithromycin showed
similar activities against non-Typhi Salmonella isolates ob-
tained from various sources, including isolates displaying re-
sistance to ceftriaxone and decreased susceptibility to cipro-
floxacin. Our findings are consistent with a study reporting
azithromycin susceptibility among non-Typhi Salmonella iso-
lates collected in Finland (17) and are also consistent with MIC
distribution data presented for Salmonella serotype Typhi and
Shigella (15, 19).
The highest MIC value observed in the present study was 32
?g/ml, and it was detected in three human isolates belonging to
serotypes Kentucky, Montevideo, and Paratyphi A. These iso-
lates were all negative for ereA, ereB, ermB, mefA, mphA,
mphB, and mphD, genes associated with macrolide resistance
(28). In addition, attempts to transfer the resistance failed in
all three isolates, indicating a lack of plasmid-mediated mech-
anisms. Other possible mechanisms include mutations in the
23S rRNA genes or the rlpD and rlpV genes, the last two of
which encode ribosomal proteins L4 and L22, respectively
(33). Further investigations will be necessary to determine
whether these isolates acquired a resistance mechanism or
whether they belong to the wild-type distribution and their
slightly elevated MICs are due to normal variation in the test-
ing methodologies. The latter possibility is supported by the
fact that these three isolates displayed MICs of 16 ?g/ml upon
retesting with Etest.
Studies investigating azithromycin resistance mechanisms in
Salmonella are scarce. Gunell and colleagues reported an rlpD
mutation among isolates with azithromycin MICs of 64 to 128
?g/ml (17). Moreover, an isolate of Salmonella enterica sero-
type Stanley collected through NARMS in the United States in
1999 displayed an azithromycin MIC of 128 ?g/ml and har-
bored the mphA gene (12; J. P. Folster et al., unpublished
Although clinical breakpoints are essential for defining clin-
ical resistance, they might not always be the most sensitive tool
for detecting isolates with an acquired resistance mechanism.
The detection of resistance mechanisms is becoming increas-
ingly important as surveillance programs recognize their role in
the global control of antimicrobial resistance. In this context,
the use of microbiological breakpoints or epidemiological cut-
off values (ECOFFs) is useful. The concept of ECOFFs was
introduced by EUCAST as a way of distinguishing bacteria
without resistance (wild type [WT]) from those with acquired
resistance (21). The WT ECOFF is expressed as ?x ?g/ml and
serves to divide the bacterial population into two groups: those
that are wild type (WT) and those that acquired resistance and
are non-wild type (NWT) (21). Based on this study and the
work of EUCAST and Gunell and coauthors, we propose an
ECOFF for WT Salmonella isolates of ?16 ?g/ml of azithro-
mycin (11, 17). This means that isolates with an MIC of ?32
?g/ml could be considered NWT for surveillance purposes.
Additional studies would be required to confirm the accuracy
of the proposed ECOFF and determine whether isolates dis-
playing an MIC value of 32 ?g/ml belong to the wild-type
Whether NWT isolates should be classified as clinically re-
sistant remains to be determined following the accumulation of
clinical endpoint data. Recently, the first case of azithromycin
treatment failure in a patient with invasive Salmonella infec-
tion was reported (26). The Salmonella serotype Paratyphi A
isolate that initially displayed an azithromycin MIC of 64 ?g/ml
later displayed an MIC of 256 ?g/ml when it was isolated from
a second blood culture (26).
In summary, the azithromycin MIC distributions for Salmo-
nella serotype Typhi and non-Typhi Salmonella isolates were
concordant, and the majority of the isolates showed MICs of 4
to 8 ?g/ml. Based on data from this study and the work of
others (11, 17), we suggest an ECOFF for WT Salmonella of
?16 ?g/ml of azithromycin. Moreover, as azithromycin use
increases for the management of invasive salmonellosis, we
encourage national and international antimicrobial susceptibil-
ity testing consensus groups to consider the MIC distribution
data presented here to contribute to the establishment of clin-
ical breakpoints for azithromycin and Salmonella enterica.
We thank the public health laboratories that participate in the
NARMS, the Retail Foods Survey Working Group, and the FSIS
laboratories for submitting the isolates.
The findings and conclusions in this report are those of the authors
and do not necessarily represent the official position of the Centers for
Disease Control and Prevention.
This work was supported by interagency agreements that the CDC
and USDA have with the FDA Center for Veterinary Medicine (FDA-
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