Characteristics of Staphylococcus aureus strains isolated from clinical and non-clinical human sources in Trinidad: susceptibility to bacteriophages and antimicrobial agents, and toxigenicity

Frequency of resistance to methicillin and other antimicrobial agents among Staphylococcus aureus strains isolated from pigs and their human handlers in Trinidad

Background

Methicillin-resistant Staphylococcus aureus (MRSA) has emerged recently worldwide in production animals, particularly pigs and veal calves, which act as reservoirs for MRSA strains for human infection. The study determined the prevalence of MRSA and other resistant strains of S. aureus isolated from the anterior nares of pigs and human handlers on pig farms in Trinidad.

Methods

Isolation of S. aureus was done by concurrently inoculating Baird-Parker agar (BPA) and Chromagar MRSA (CHROM) with swab samples and isolates were identified using standard methods. Suspect MRSA isolates from Chromagar and BPA were subjected to confirmatory test using Oxoid PBP2 latex agglutination test. The disc diffusion method was used to determine resistance to antimicrobial agents.

Results

The frequency of isolation of MRSA was 2.1% (15 of 723) for pigs but 0.0% (0 of 72) for humans. Generally, for isolates of S. aureus from humans there was a high frequency of resistance compared with those from pigs, which had moderate resistance to the following antimicrobials: penicillin G (54.5%, 51.5%), ampicillin (59.1%, 49.5%), and streptomycin (59.1%, 37.1%), respectively. There was moderate resistance to tetracycline (36.4%, 41.2%) and gentamycin (27.2%, 23.7%) for human and pig S. aureus isolates, respectively, and low resistance to sulfamethoxazole-trimethoprim (4.5%, 6.2%) and norfloxacin (9.1%, 12.4%), respectively. The frequency of resistance to oxacillin by the disc method was 36.4 and 34.0% from S. aureus isolates from humans and pigs, respectively. Out of a total of 78 isolates of S. aureus from both human and pig sources that were resistant to oxacillin by the disc diffusion method, only 15 (19.2%) were confirmed as MRSA by the PBP'2 latex test kit.

Conclusions

The detection of MRSA strains in pigs, albeit at a low frequency, coupled with a high frequency of resistance to commonly used antimicrobial agents in pig and humans could have zoonotic and therapeutic implications. Finally, the diagnostic limitation of using CHROMagar and testing for oxacillin resistance by the disc diffusion method alone to determine MRSA strains without performing confirmatory tests cannot be overemphasized because the possibility of overdiagnosis of MRSA infections cannot be ignored.

Bacteremia in children at a regional hospital in Trinidad

OBJECTIVE

The objective of this study was to provide a Trinidadian perspective on pediatric community-acquired and hospital-acquired bacteremia via the documentation of common etiologic agents, antimicrobial profiles of the isolated pathogens, and patient outcome.

METHODS

This was a six-year retrospective study of children with bacteremia admitted to the pediatric wards of the San Fernando General Hospital, Trinidad.

RESULTS

Seven hundred and four episodes of pediatric bacteremia were reviewed during the six-year study period. The predominant isolate was Staphylococcus aureus (23.9%), followed by Pseudomonas aeruginosa (15.5%), Klebsiella pneumoniae (12.5%), and Enterobacter spp (11.1%). The remaining isolates each accounted for less than 10% of total isolates. The mortality rate was highest for P. aeruginosa (39.4%), Streptococcus pneumoniae (22.5%), and Escherichia coli (19.2%). Of the six cases due to Neisseria meningitidis, only two survived. The overall mortality rate for the study period was 15.1%, but varied considerably according to age. All deaths due to P. aeruginosa and E. coli occurred in neonates. Almost 90% and about half of all S. aureus were resistant to ampicillin and erythromycin, respectively; nineteen (11.3%) were methicillin-resistant. More than 95% of K. pneumoniae and more than 87% of Haemophilus influenzae were resistant to ampicillin. Group B streptococci were fully susceptible to ampicillin and amoxicillin-clavulanic acid, but showed >90% and >70% resistance to tetracycline and trimethoprim-sulfamethoxazole, respectively. Of the 40 strains of S. pneumoniae isolated, 10.0% had a minimum inhibitory concentration (MIC) >or=4 microg/mL (resistance) and 12.5% had a MIC=2.0 microg/mL (intermediate resistance) to ceftriaxone, while 7.5% showed intermediate resistance (MIC between 0.12 and 1 microg/mL) and 25.0% showed resistance (MIC >or=2 microg/mL) to penicillin.

CONCLUSION

The bacteremia rate was found to be 8.4% among hospitalized children suspected of having sepsis and from whom a blood culture was positive. Bacteremia was also associated with a high mortality rate of 15.1%. An unusually high level of bacteremia with Gram-negative enteric bacteria was seen, which might indicate cross infection and reflect a breakdown in infection control measures. Relatively high-level resistance of S. pneumoniae to penicillin and ceftriaxone was not seen, even though the overall prevalence of resistance to other antibiotics among other pathogens was relatively low. The drugs that showed the greatest efficacy were imipenem, gentamicin, ciprofloxacin, and the cephalosporins ceftazidime and ceftriaxone.

Prevalence and antimicrobial susceptibility pattern of methicillin resistant Staphylococcus aureus isolates from Trinidad & Tobago

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) has become increasingly prevalent worldwide since it was first reported in a British hospital. The prevalence however, varies markedly in hospitals in the same country, and from one country to another. We therefore sought to document comprehensively the prevalence and antimicrobial susceptibility pattern of MRSA isolates in Trinidad and Tobago.

METHODS

All Staphylococcus aureus isolates encountered in routine clinical specimens received at major hospitals in the country between 2000 and 2001 were identified morphologically and biochemically by standard laboratory procedures including latex agglutination test (Staphaurex Plus; Murex Diagnostics Ltd; Dartford, England); tube coagulase test with rabbit plasma (Becton, Dickinson & Co; Sparks, MD, USA), and DNase test using DNase agar (Oxoid Ltd; Basingstoke, Hampshire, England). MRSA screening was performed using Mueller-Hinton agar containing 6 mug oxacillin and 4% NaCl, latex agglutination test (Denka Seiken Co. Ltd, Tokyo, Japan) and E-test system (AB Biodisk, Solna, Sweden). Susceptibility to antimicrobial agents was determined by the modified Kirby Bauer disc diffusion method while methicillin MICs were determined with E-test system.

RESULTS

Of 1,912 S. aureus isolates received, 12.8% were methicillin (oxacillin) resistant. Majority of the isolates were recovered from wound swabs (86.9%) and the least in urine (0.4%) specimens. Highest number of isolates was encountered in the surgical (62.3%) and the least from obstetrics and gynaecology (1.6%) facilities respectively. Large proportions of methicillin sensitive isolates are >85% sensitive to commonly used and available antimicrobials in the country. All MRSA isolates were resistant to ceftriaxone, erythromycin, gentamicin and penicillin but were 100% sensitive to vancomycin, rifampin and chloramphenicol.

CONCLUSION

There is a progressive increase in MRSA prevalence in the country but the present rate is still low in comparison to values in some other countries. Vancomycin is still the drug of choice for treating multidrug resistant MRSA infections. Further use of molecular studies to monitor the epidemiology of MRSA in these hospitals in the country is highly recommended too.

Methicillin-resistant Staphylococcus aureus prevalence: current susceptibility patterns in Trinidad

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) has become one of the most widespread causes of nosocomial infections worldwide. Recently, reports have emerged that S. aureus strains recovered from community-acquired infections are also methicillin-resistant. This study was undertaken to analyze the prevalence of methicillin resistance among isolates at a regional hospital in Trinidad, and document the current resistance profile of MRSA and methicillin-sensitive Staphylococcus aureus (MSSA) to the commonly used anti-staphylococcal agents.

METHODS

Over a 6-year period we analyzed 2430 isolates of S. aureus strains recovered from various clinical sources, from hospital and community practices. Antimicrobial susceptibility testing was done according to guideline recommendations of the National Committee for Clinical Laboratory Standards.

RESULTS

The prevalence of MRSA from surgical/burn wounds, urine and pus/abscess were 60.1%, 15.5% and 6.6%, respectively. The major sources of MSSA were surgical/burn wounds, pus/abscess and upper respiratory tract specimens with rates of 32.9%, 17.1% and 14.3%, respectively. The greatest prevalence of resistance of MRSA was seen for erythromycin (86.7%), and clindamycin (75.3%). Resistance rates among MSSA were highest for ampicillin (70%). Resistance rates for tetracycline were similar among both MRSA (78.7%) and MSSA (73.5%). The MRSA recovery rates from nosocomial sources (20.8%) was significantly higher than that of previous years (12.5%) (p < 0.001), whereas rates among community isolates were relatively similar for the same period (4.1% versus 8.1%).

CONCLUSION

The prevalence of MRSA in the hospital increased from 12.5% in 1999 to 20.8% in 2004. Most isolates were associated with infected surgical/burn wounds which may have become infected via the hands of HCPs during dressing exercises. Infection control measures aimed at the proper hand hygiene procedures may interrupt the spread of MRSA. HCPs may also be carriers of MRSA in their anterior nares. Surveillance cultures of both patients and HCPs may help to identify carriers who would be offered antibiotics to eradicate the organisms. Most MRSA are resistant to several non-beta-lactam antibiotics. Frequent monitoring of susceptibility patterns of MRSA and the formulation of a definite antibiotic policy maybe helpful in decreasing the incidence of MRSA infection.

PCR detection of toxic shock syndrome toxin of Staphylococcus aureus from Tripoli, Libya

Sixty-three Staphylococcus aureus strains (40 from clinical sources and 23 from food sources) were examined for toxic shock syndrome toxin-1 (TSST-1) using PCR, phage typed using the international phage set (IPS) and tested for their susceptibility to antibiotics. Only three strains (all from clinical sources) were positive for the TSST-1 gene (tst). The majority of S. aureus strains that were typeable by IPS belonged to group II. Resistance to one or more antibiotics was detected in 47.5 and 73.9 % of clinical and food strains, respectively. This is the first time that PCR detection of tst in S. aureus has been reported from Libya, and further studies are needed on the occurrence of toxic shock syndrome in the community and the role of TSST-1-producing S. aureus in this disease in Libya.

Relatedness of Staphylococcus aureus strains isolated from milk and human handlers in dairy farms in Trinidad

The susceptibility of S. aureus strains isolated from milk and human handlers in dairy farms in Trinidad to bacteriophages was determined. Of the 110 strains isolated from bulk milk, 73 (66.4%) were typable either at routine test dilution (RTD) or 100-RDT compared to 108 (72.5%) of 149 strains from composite milk sensitive but the difference was not statistically significant (P > or = 0.05; chi 2). Strains of S. aureus from human handlers had significantly (P < or = 0.05; chi 2) lower sensitivity, with only 90 (59.6%) of 151 strains typable. Phages in various groups were predominantly more active on both milk isolates, 54.1% (98 of 181) and human strains, 73.3% (66 of 90). Phage 42D alone lysed 22 (12.2%) but with other phages typed 90 (49.7%) of 181 typable strains from milk compared to a lytic activity of only 1.1% (1 of 90) alone and 35 (38.9%) with other phages for strains isolated from human handlers. The differences were however not statistically significant (P > or = 0.05; chi 2). Relatedness of S. aureus strains isolated from bulk milk, composite milk and human handlers on each farm was demonstrated in 21 (46.7%) of 45 dairy farms using the phage patterns detected. Similarly, the relatedness of S. aureus strains isolated from the anterior nares and hands of each human milker was detected on 10 (35.7%) of 28 dairy farms studied. It was concluded that S. aureus strains from human milkers contaminated both composite and bulk milk on the farms studied while bovine strains of S. aureus were also acquired by humans during milking.