Bacteria: Antibiotic resistance in S. aureus

${\text{MLS}}_B$‍ resistance refers to resistance to macrolide, lincosamide and streptogramin B antibiotics (referred to as the ${\text{MLS}}_B$‍ antibiotics) in Staphylococcus aureus bacteria. It results from the production of a methylating enzyme (methylase) that is encoded by erythromycin ribosome methylation (erm) genes. This enzyme installs methyl groups on bacterial ribosomal RNA (rRNA), thereby altering the ribosomal binding site of ${\text{MLS}}_B$‍ antibiotics. The enzyme can be constitutively expressed or induced by the administration of macrolides such as erythromycin, which is a strong inducer of the methylase, or clindamycin, which is a weak inducer.

Inducible resistance results from the binding of macrolides to specific upstream translational attenuator sequences, leading to changes in mRNA secondary structure that allow the coding region to be processed by ribosomes. In constitutive resistance, alterations to upstream attenuator sequences – including deletions, duplications, and other mutations – lead to constitutive expression of the methylase gene.

An in vitro test for inducible ${\text{MLS}}_B$‍ resistance is the “D test”. In this test, erythromycin- and clindamycin-impregnated disks are placed 15–20 mm apart on a “lawn” of S. aureus. In a positive D test, the zone of clearance (the area of inhibited bacterial growth) around the disk is blunted, creating a D shape (Figure 1).

Figure 1 Negative and positive clindamycin D tests

Researchers performed a D test on 100 distinct clinical isolates of Staphylococcus aureus and observed different morphologies of the zone of clearance, as shown in Figure 2.

Figure 2 D test zone-of-clearance morphologies in six distinct staphylococcal isolates Image adapted from Steward et al. J Clin Microbiol. 2005 Apr;43(4):1716-21.