Bioactivity and molecular docking studies of selected plant compounds

BACKGROUND

The shortcomings of synthetic pesticides are becoming increasingly evident. In the pursuit of looking for alternative, environmentally friendly pesticides, the potential use of two naturally occurring sesquiterpenes, α-bisabolol and bisabolol oxide A, as bactericides or acaricides, was investigated against three gram-positive and three gram-negative bacteria, and the eggs and larvae of the cattle tick Hyalomma scupense, respectively. The primary targeted site of synthetic insecticides is acetylcholinesterase (AChE), so the inhibitory activity of both sesquiterpenes was evaluated. Then, molecular docking and molecular mechanics generalized Born surface area (MM-GBSA) studies were performed to gain insight into their binding mode in the AChE active site.

RESULTS

Bisabolol oxide A showed good activity against Staphylococcus aureus, Escherichia coli, and Salmonella enteritidis with inhibition zones ranging from 16 to 34 mm and minimal inhibitory concentration (CIM) of 3.125  mg mL-1. Ovicidal and larvicidal assays revealed promising activity of α-bisabolol and bisabolol oxide A against H. scupense, with bisabolol oxide A being more effective against larval ticks with lethal concentration (LC50) value of 0.78%. Both compounds also displayed potent anti-AChE activity with inhibition concentration (IC50) values of 37.09 and 28.14 μg mL-1, respectively. Furthermore, α-bisabolol and bisabolol oxide A exhibited good and comparable docking scores (-7.289 and -7.139 Kcal mol-1, respectively) and were found to accommodate in the active-site gorge of AChE via hydrogen bonding and hydrophobic interactions.

CONCLUSION

Bisabolol oxide A and, to a lesser degree, α-bisabolol are active against bacteria and ectoparasites and may represent an economical and sustainable alternative to toxic synthetic pesticides to control pathogens. © 2024 Society of Chemical Industry.

Macrolide and tetracycline resistance in clinical strains of Streptococcus agalactiae isolated in Tunisia

Between 2007 and 2009, 226 clinical strains of Streptococcus agalactiae, recovered from female genital specimens and from gastric fluid or ear specimens from infected newborns, were isolated at the Laboratory of Microbiology of Charles Nicolle Hospital of Tunis. They were investigated to determine the prevalence of antibiotic resistance and to characterize the mechanisms of resistance to macrolide and tetracycline. All strains were susceptible to penicillin, ampicillin and quinupristin-dalfopristin. They were resistant to chloramphenicol (3.1%), rifampicin (19.1%), erythromycin (40%) and tetracycline (97.3%); 3.1% were highly resistant to streptomycin and 1.3% to gentamicin. Among the erythromycin-resistant isolates, 78.7% showed a constitutive macrolide-lincosamide-streptogramin B (MLS(B)) phenotype with high-level resistance to macrolides and clindamycin (MIC(50) >256 µg ml(-1)), 10% showed an inducible MLS(B) phenotype with high MICs of macrolides (MIC(50) >256 µg ml(-1)) and low MICs of clindamycin (MIC(50)=8 µg ml(-1)) and 2.2% showed an M phenotype with a low erythromycin-resistance level (MIC range=12-32 µg ml(-1)) and low MICs of clindamycin (MIC range: 0.75-1 µg ml(-1)). All strains were susceptible to quinupristin-dalfopristin and linezolid (MIC(90): 0.75 µg ml(-1) for each). MLS(B) phenotypes were genotypically confirmed by the presence of the erm(B) gene and the M phenotype by the mef(A) gene. Resistance to tetracycline was mainly due to the tet(M) gene (93.1%) encoding a ribosome protection mechanism. This determinant is commonly associated with the conjugative transposon Tn916 (P≤0.0002). tet(O) and tet(T) existed in a minority (2.2% and 0.4%, respectively). The efflux mechanism presented by tet(L) was less frequently present (4.5%). No significant association was found between erm(B) and tet(M) genes.

Epidemiological markers of Streptococcus pyogenes strains in Tunisia

To further understand the epidemiology of Streptococcus pyogenes or group A streptococcus (GAS) infections in Tunisia, phenotypic and genomic markers of GAS isolates, including antibiotic susceptibility, biotypes, T and emm types and toxin gene profiles, have been characterized. A total of 103 isolates, collected between 2000 and 2006, were investigated; 47 were recovered from invasive infections, and 56 from non-invasive infections. Rates of resistance to tetracycline, erythromycin, clindamycin and rifampin were 70.8%, 4.8%, 4.8% and 0.9%, respectively. High levels of resistance to streptomycin and kanamycin were observed in 1.9% and 4.8% of isolates, respectively. Biotype 3 was most common. Twenty different T patterns were observed, with a predominance of T3/13/B3264, and 38 different emm types. In both invasive and non-invasive isolates, emm118 (9.7%), emm42 (8.7%), emm1 (7.8%), st432 (6.8%), emm28 (5.8%) and emm76 (5.8%) were the most prevalent types; emm1, emm76 and emm18 were mainly observed among invasive infections, whereas emm118 (12.5%), emm42 (10.7%) and emm28 (8.9%) were predominant among non-invasive infections. The speB gene was detected in all isolates, but there were variable frequencies of speA, speC and ssa (20.3%, 32% and 25.2% respectively). Significant associations of emm1, emm18 and emm3 with speA and of emm4 and st432 with ssa were found. This first report from Tunisia revealed a unique emm distribution of GAS that differs from those of other regions. This information on the distribution of such emm types will be useful for the development of an appropriate vaccine in a country where the incidence of rheumatic fever remains high.

High rates of macrolide resistance among clinical isolates of Streptococcus agalactiae in Tunisia

One hundred sixty non duplicate erythromycin resistant Streptococcus agalactiae isolates were collected in Tunisia from January 2005 to December 2007 They were investigated to determine their resistance level to different macrolides and the mechanisms involved. Most erythromycin resistant S. agalactiae isolates were isolated from urinary specimens (38.75%, 62/160). The constitutive MLSB phenotype (cMLS) showed in 84.3% (135/160) with high MICs of macrolides and lincosamides (MIC90>256 microg/mL) and 8.2% (13/160) inducible MLSB phenotype (iMLS) with high MICs of macrolides (MIC90>256 microg/mL) and moderately increased MICs of lincosamides (MIC90=8 microg/mL). The M phenotype showed in 7.5% (12/160) with moderately increased MICs of macrolides (MIC90=32 microg/mL) and low MICs of lincosamides (MIC90=0.75 microg/mL). All strains were susceptible to quinupristun-dalfopristin association and linezolid (MIC90: 05 and 0.38 microg/mL respectively). Strains with MLSB phenotype harboured erm(B) gene with 825% (n=132), erm(TR) gene with 8.12% (n=13) and erm(B) plus mef (A) with 1.88% (n=3). All strains categorized as M phenotype carried the mef(A) gene (75%, n=12). cMLSB phenotype conferring cross resistance to macrolides, lincosamides and streptogramins B with high level of resistance was the most prevalent.