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Year : 2017  |  Volume : 35  |  Issue : 3  |  Page : 415--416

Survivability and fitness cost of heterogeneous vancomycin-intermediate Staphylococcus aureus

Avinash Singh, Sanjay Singh, Jyoti Singh, Mohibur Rahman, Ashutosh Pathak, Kashi Nath Prasad 
 Department of Microbiology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, Uttar Pradesh, India

Correspondence Address:
Kashi Nath Prasad
Department of Microbiology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow - 226 014, Uttar Pradesh
India

Abstract

The aim of this study was to observe the survivability and fitness cost of heterogeneous vancomycin-intermediate Staphylococcus aureus(hVISA) isolates. Survivability study was performed on dry cotton swab, and fitness cost was evaluated by estimating growth kinetics and generation time constant in BACTEC automated system. Total mean maximum time of recovery on primary culture was 4.1 and 7.1 weeks (P = 0.0001) for hVISA and vancomycin-sensitive S. aureus (VSSA), respectively, in dry starved condition. No significant difference between the mean value of lag phase duration (P = 0.89) was noted between hVISA and VSSA isolate in growth kinetics. However, we observed lesser generation time of hVISA isolates compared to S. aureus ATCC 29213 (P = 0.0076). This study concluded that a significant difference in generation time between VSSA and hVISA and suggests that hVISA have fitness cost compared to VSSA. However, further studies with more cases are required.

How to cite this article:
Singh A, Singh S, Singh J, Rahman M, Pathak A, Prasad KN. Survivability and fitness cost of heterogeneous vancomycin-intermediate Staphylococcus aureus.Indian J Med Microbiol 2017;35:415-416

How to cite this URL:
Singh A, Singh S, Singh J, Rahman M, Pathak A, Prasad KN. Survivability and fitness cost of heterogeneous vancomycin-intermediate Staphylococcus aureus. Indian J Med Microbiol [serial online] 2017 [cited 2020 May 31 ];35:415-416
Available from: http://www.ijmm.org/text.asp?2017/35/3/415/216630

Full Text

 Introduction



Global concerns have been raised due to emergence of heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA). hVISA are defined as the strains of S. aureus that are sensitive to vancomycin but contains vancomycin-intermediate subpopulations.[1] hVISA strains are slow grower and on culture mixture of small and large colonies are seen.[2] We reported gradual increasing the prevalence of hVISA at our centre from 2012 to 2015.[3] Association of vancomycin treatment failure is also reported with hVISA isolates.[1] Therefore, study on survivability and fitness cost of hospital isolates of hVISA isolates is needed to reduce such type of infections in hospitals.

To determine the survivability under dry starvation condition, we included five strains; two clinical hVISA isolates, Mu3 (hVISA prototype) and Mu50 (VISA prototype) as positive controls and vancomycin-sensitive S. aureus ATCC 29213 (VSSA) as negative control. All the strains were previously characterised[4] and details are given in [Table 1]. Survivability experiment was carried out on sterile cotton swab at room temperature.[5] Briefly, five colonies of each isolates were picked up on cotton swabs, inserted into 15 ml sterile falcon tube, and incubated at room temperature in a sterile box. On weekly basis, the swabs were inoculated on to blood agar plates and incubated at 37°C for 48 h. This was repeated until cultures showed no growth.{Table 1}

Fitness cost of hVISA isolates were studied by comparing the length of lag phase during growth curve.[5] The lag phase duration was taken to be the beginning of the maximum growth rate. We also attempted to see the fitness of hVISA by automated BACTEC-9120 (BD Biosciences, USA) culture method following previously designed protocol.[6] The length of time to culture positive was measured for all the strains. The growth rate constant k was determined using the equation k = (logA-logB)/t, where A was the largest inoculum employed, B was the smallest inoculum, and t was the difference in time to positivity in hours. This experiment was repeated three times at different time periods.

The experiment on survivability consistently indicated that hVISA could survive for several weeks on dry cotton swabs at room temperature but lesser than VSSA isolate [Table 1]. Total mean maximum time of recovery on primary culture was 4.1 and 7.1 weeks (P = 0.0001) for hVISA and VSSA, respectively. However, the mechanism behind lesser survivability of hVISA still remains unclear. Our result differs with the previous report where no consistent difference in survivability between methicillin-resistant S. aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) inoculated onto the two cotton surfaces were tested.[7]

Growth kinetics experiment showed lag phase duration of hVISA was higher (3.25 h) than VSSA (2.25 h) by growth kinetics experiment [Table 1]. However, no significant difference between the mean value of lag phase duration (P = 0.89) was noted between hVISA and VSSA isolate. This result indicates fitness cost associated with hVISA isolates but the difference was not significant. We observed almost similar results in all three times. We observed, lesser generation time of hVISA isolates compared to S. aureus ATCC 29213 [P = 0.0076; [Table 1]. Antibiotic resistance mechanism might have contributed to fitness cost that resulted in reduced bacterial growth corroborating earlier report.[8] However, no significant difference in generation time between hVISA and MRSA was observed [Table 1].

Our study has a limitation that we tested only a small number of non-isogenic clinical isolates, therefore, the relative fitness of each in relation to observed resistance is difficult to assess. However, interestingly, this study showed a significant difference in generation time between MSSA and MRSA. This study suggests that hVISA have fitness cost compared to VSSA. However, further studies with more cases are required.

Acknowledgement

Author thanks to Ms. Malay Ghar (Sr. Technician Department of Microbiology, SGPGIMS, Lucknow) for providing the support in performing experiment in BACTEC automated system.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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