This prospective study was conducted in the Department of Microbiology over a period of two months from June 2019 to July 2019 as part of ICMR-STS 2019 (Reference ID.2019-02280) Clearance was obtained from the Institutional Ethics Committee (INST.EC/EC/057/2019-20).

Fifty non-duplicate S. aureus strains isolated from various clinical samples were included in the study. The strains were streaked on nutrient agar plates after thawing the vials. A smear was prepared from an isolated colony and stained with Gram’s stain [Table/Fig-1]. Biochemical tests like catalase test and tube coagulase test were done to reconfirm the identity of the strain. Gram positive cocci which were catalase positive and tube coagulase positive [Table/Fig-2] were identified as Staphylococcus aureus.

[Table/Fig-1]:

Gram positive cocci in singles, pairs and clusters.

(magnification- X1000)

[Table/Fig-2]:

Tube coagulase test. The arrow showing positive tube coagulase yielded by the test isolate.

NC: Negative control, PC: Positive control; T: Test isolate

Screening for MRSA: Screening for methicillin resistance was done by modified Kirby Bauer disc diffusion method using cefoxitin (30 μg) discs [16]. Inoculum for lawn culture was prepared by direct colony suspension method. Three to five isolated colonies of S. aureus were suspended in 5 mL of peptone water and the turbidity of the test suspension was standardised to match 0.5 McFarland standard. A lawn culture of the test organism was made on Mueller Hinton agar plates according to standard protocols. Cefoxitin disc (30 μg) was placed on the lawn culture and the plates were incubated at 35°C for 16-18 hours. The diameter of the zone of inhibition was measured using a ruler. A zone size of ≥22 mm was interpreted as methicillin sensitive and ≤21 mm was interpreted as methicillin resistant as per Clinical and Laboratory Standards Institute (CLSI) guidelines. S. aureus American Type Culture Collection (ATCC) 25923 and S. aureus ATCC 43300 were used as controls [16].

Ceftaroline susceptibility: Testing for ceftaroline susceptibility was done by E-strip method. The ceftaroline E strips 0.002-32 μg/mL was obtained from Biomerieux, France. Inoculum preparation and lawn culture of the test organism was done as detailed for disc diffusion method. The E-strips were placed on the lawn culture and the plates were incubated at 37°C for 18-24 hours. MIC’s were read where the ellipse intersects the MIC scale. Since E-strip has continuous gradient, MIC values “in-between” two-fold dilutions can be obtained. These values were rounded up to next two-fold dilution before categorisation. MIC ≤1 μg/mL, 2-4 μg/mL and ≥8 μg/mL were interpreted as ceftaroline susceptible, SDD and ceftaroline resistant respectively as per CLSI guidelines. S. aureus ATCC 29213 was used as control [16].

Descriptive analysis of the distribution of sample, age, gender, and antimicrobial susceptibility data were done, and the results obtained were presented as frequencies and percentages.

Among the 50 S.aureus isolates 28 (56%) were isolated from male patients and 22 (44%) from females. The age range of patients from whom the S.aureus was isolated was 2-73 years, mean age being 41 years.

Of the 50 non-duplicate S. aureus isolates, 47 (94%) were isolated from pus, two (4%) from blood and one (2%) from endotracheal aspirate.

Among the 50 isolates tested 28 (56%) were methicillin resistant and 22 (44%) were methicillin sensitive. The age and gender distribution for MRSA and MSSA are shown in [Table/Fig-3].

All the S. aureus isolates were found to be susceptible to ceftaroline with their MIC’s ranging from 0.064 to 0.5 μg/mL. Frequency distribution of ceftaroline MIC’s of MSSA and MRSA isolates are shown in [Table/Fig-4]. It was also observed that the MSSA isolates had lower MIC’s when compared to MRSA. Majority (90%) of MSSA isolated had an MIC of ≤0.25 μg/mL in comparison to 13 (46.43%) of MRSA isolates which had an MIC of ≤0.25 μg/mL. A MIC of 0.5 μg/mL was observed among 15 (53.57%) of the MRSA isolates which included 14 isolates having MIC of 0.38 μg/mL rounded-up to 0.5 μg/mL.

A MRSA isolate with ceftaroline MIC of 0.25 μg/mL is shown in [Table/Fig-5].

[Table/Fig-5]:

MRSA isolate with ceftaroline MIC of 0.25 μg/mL.

Staphylococcus aureus is an important cause of hospital as well as community acquired infections. In the era of increasing antimicrobial resistance, treating the infections caused by MRSA is posing a real challenge to the clinicians. Emergence of multi-drug resistant MRSA isolates further complicates the treatment of infections caused by these organisms [13]. Ceftaroline fosamil has been approved as an alternative for a severe MRSA infection [8].

In this study, all the 50 clinical isolates of S. aureus, inclusive of 22 MSSA and 28 MRSA strains were found to be susceptible to ceftaroline. This corelates with the findings of an Indian study, where all the 50 MRSA isolates isolated from various clinical samples were found to be sensitive to ceftaroline [13]. Similar results were also observed in a multi-centre study from Spain, where the all S. aureus isolates tested were inhibited by ceftaroline with a MIC of ≤1 μg/mL [17]. The antimicrobial resistance surveillance program, Assessing Worldwide Antimicrobial Resistance and Evaluation (AWARE), which evaluated the trends in S. aureus susceptibility rates to ceftaroline, reported a 100% susceptibility to ceftaroline among MSSA. However, the study found that the susceptibility of MRSA to ceftaroline decreased marginally from 99.4% in 2010 to 98.6% in 2016 [18].

This study found that the MSSA isolates had much lower ceftaroline MIC’s when compared to MRSA isolates. Majority (90.9%) of MSSA isolates had an MIC ≤0.25 μg/mL. This correlates with the findings from a multi-centric study from India [14]. A multi-centric study from Latin American countries, as part of AWARE surveillance program, also reported a similar finding with 98.3% of MSSA isolates having a ceftaroline MIC of ≤0.25 μg/mL [19].

All the MRSA isolates in this study were found to be susceptible to ceftaroline with 0.5 μg/mL being the highest ceftaroline MIC detected. This is in contrast to other studies from India which have reported a higher ceftaroline MIC. Further details are shown in [Table/Fig-6] [14,15].

One such study showed that 6% and 2% of tested S. aureus had a ceftaroline MIC of 2 μg/mL and 4 μg/mL, respectively [14], which according to the old Clinical and Laboratory Standards Institute (CLSI) guidelines were interpreted as intermediate and resistant respectively. If the current CLSI guidelines are applied, these isolates will be classified as SDD and not as resistant [14,16]. The authors of another study from Maharashtra, India reported that 93.33% MRSA isolates were susceptible to ceftaroline with the MIC being 0.75 μg/mL and concluded that it can be considered an effective alternative treatment, while vancomycin and linezolid can be kept as reserve drug [15]. A multicentric study across seven provinces in Turkey found that 94.3% of tested MRSA isolates were inhibited by ceftaroline (MIC≤1 μg/mL) [20]. In a study conducted in the US hospitals from 2008-2011, the authors found that all daptomycin non-susceptible Staphylococci isolates, 85.7% and 91.9% of linezolid-resistant S.aureus isolates and S.aureus isolates with a vancomycin MIC of ≥2 μg/mL respectively were susceptible to ceftaroline. The authors concluded that ceftaroline may be considered as a valuable treatment option for infections caused by multidrug resistant S. aureus [21].

The recommended dosage of ceftaroline is 600 mg administered every 12 hours by intravenous (IV) infusion over 60 minutes in patient’s ≥18 years of age [8]. Apart from the clinical efficacy, it is important to consider the adverse effects as well while prescribing any drug. The most common adverse effects with ceftaroline includes nausea, vomiting and diarrhoea. The incidence of which is 3-5% which was comparable with vancomycin/aztreonam with a dosing regimen of vancomycin 1g every 12 hours plus aztreonam 1g every eighth hourly [9]. Clinical trials have also shown that ceftaroline is as effective as ceftriaxone, and combination of vancomycin/aztreonam for the treatment of community-acquired pneumonia and complicated SSI, respectively [22].

This study has evaluated a small number of isolates, as it was an ICMR-STS project with a limited study duration of two months. Prospective studies with larger sample size are warranted to support or verify the findings. Secondly, the present study has not evaluated resistance of MRSA isolates to other antibiotics like vancomycin, linezolid, teicoplanin which may be considered as a limitation of this study.

Taking into consideration, the high susceptibility rates and comparable or better tolerance of patients to ceftaroline, when compared to vancomycin, for the treatment of infections caused by MRSA isolates, ceftaroline can be considered as an effective alternative for treatment of infections caused by MRSA. But like most drugs, ceftaroline might become ineffective, if misused. With the increasing resistance to antibiotics and very few newer antibiotics in the pipeline, it is high time to stop misusing the antibiotics in order to help in combating the development of further resistance and prevent going back to the pre-antibiotic era.