The prevalence of antimicrobial resistance is ever increasing. This rising abundance occurring simultaneously in animal, human, and environmental microbial communities, coupled with the easy transmissibility resistance among species and the paucity of novel antibiotics in the pipeline creates the consonant antimicrobial resistance threat. We'll focus on antimicrobial resistance in the next two modules. First, we'll discuss three gram-positive organisms in which resistance is common. Staphylococcus aureus, Enterococcus species, and Streptococcus pneumoniae. We'll subsequently address multi-drug resistance and gram-negative aerobic bacilli, focusing on resistance to beta-lactam antibiotics in the next module. First, some definitions. Bacteria primarily acquire antimicrobial resistance by one of two mechanisms; genetic mutations together with selective pressure or horizontal gene transfer. Genetic mutations can become dominant among a bacterial population as a result of selective pressure exerted by antibiotic exposure. Genetic diversity resides within every population of bacteria including subpopulations that develop resistance to certain antibiotics, often by accumulating random mutations. Upon exposure to an antibiotic, susceptible populations die, while resistant populations may survive and multiply. Resistance is then vertically passed on to daughter cells creating a resistant population. In the case of horizontal gene transfer, bacteria acquire resistance from another microorganism. Antibiotic resistance genes are carried on mobile genetic elements such as plasmids or transposons that can act as vectors transferring resistant genes to other members of the same bacterial species, as well as to bacteria in another gene is. We'll point out how specific species have acquired resistance in these two modules. It's important to remember that resistance to a variety of antibiotics may accumulate over time leading to the emergence of multi-drug resistant organisms. Although the incidence of Methicillin-resistant Staphylococcus aureus or MRSA infections in the United States have declined since the early 2000s. It remains a serious global public health threat. In a 2013 report, the CDC included both Methicillin-resistant and Vancomycin-resistant Staph aureus in the top 18 drug-resistant threats. They estimated that over 80,000 severe infections and over 11,000 deaths are due to MRSA per year in the US. Reflecting the global reach of this problem, in 2017 the WHO named Methicillin-resistant, Vancomycin intermediate or Vancomycin-resistant Staph aureus as a high priority pathogen requiring the development of new agents. MRSA remains one of the most common healthcare-associated infections in the US, although the incidence of these infections has declined by 30 percent from 2005-2011. In unknown, but likely greater number of less invasive infections occur in both the hospital and the community. MRSA was first detected in Britain in 1961, the year following the introduction of Methicillin, the first semi-synthetic penicillinase-resistant penicillin into clinical practice. MRSA has subsequently spread worldwide. Almost all Methicillin resistance is due to the mecA gene. Although resistance due to the mecC gene has been reported primarily in Europe. The mecA gene is part of a mobile genetic element called the Staphylococcal Cassette Chromosome mec. Evidence suggests that MRSA acquired the mecA gene via horizontal gene transfer from coagulase-negative Staphylococcus species. Community-acquired MRSA is highly associated with a unique mobile genetic element called mec type IV, which is less likely to carry other antimicrobial resistance genes in a smaller in size perhaps allowing for enhanced mobility. Beta-lactam antibiotics target penicillin-binding proteins, membrane-bound enzymes that catalyze the reaction necessary for cross linkage of peptidoglycan chains. The mecA gene encodes a variant penicillin-binding protein 2a, which has a low affinity for most beta-lactam antibiotics. Penicillin-binding protein 2a substitutes for other penicillin-binding proteins conferring resistance by modifying the drug target. Two cephalosporins, ceftobiprole and ceftaroline have high affinity for penicillin-binding protein 2a and as a consequence are active against MRSA. Resistance of MRSA to ceftaroline has been associated with mutations in the gene encoding penicillin-binding protein 2a. Vancomycin-resistant Staph aureus infections are extremely rare. However, because Vancomycin remains the drug of choice for most MRSA infections, resistance to this drug significantly limits available therapeutic options. There are two major mechanisms that confer Vancomycin resistance. Vancomycin intermediate resistant Staph aureus or VISA was first reported in Japan in 1997 and has spread worldwide. The majority of VISA cases emerge in patients infected with MRSA who are receiving Vancomycin therapy. Decreased susceptibilities conferred by the emergence of chromosomal mutations that altered the cell wall rendering Vancomycin less effective. According to the CLSI guidelines, MRSA isolates with Vancomycin MIC that are less than or equal to 2 micrograms per milliliter are considered susceptible, while isolates with MIC is between 4-8 are intermediate and those with MIC is greater than 16 are resistant. Some studies have suggested that a Vancomycin MIC that is equal to 2 micrograms per milliliter is associated with an increased risk of treatment failure, but recent evidence does not support this conclusion. Vancomycin-resistant Staph aureus or VRSA was first described in 2002 in the US. These strains acquired plasmid-borne copies of transposon Tn1546 via horizontal gene transfer from Vancomycin-resistant Enterococcus faecalis. This transposon encodes for genes that alter Vancomycin's target and we'll discuss this mechanism shortly. Infections due to Vancomycin-resistant Enterococcus or VRE have become increasingly prevalent, especially among Enterococcus faecium. In the 2013 report, the CDC estimates that there are 66,000 healthcare-associated infections due to VRE per year in the US. In 2017, the WHO included Vancomycin-resistant Enterococcus faecium along with MRSA as a high priority pathogen requiring the development of new agents. Treatment of infections due to Enterococcus species can be challenging since these bugs are intrinsically resistant to several classes of antibiotics including beta-lactams and aminoglycosides, and they can also acquire resistance to other classes. Two species, Enterococcus faecalis and Enterococcus faecium, cause the majority of human disease. Resistance is more common in Enterococcus faecium isolates. VRE was first described in the 1980's after Vancomycin use expanded in response to the emergence of MRSA. Vancomycin resistance in Enterococcus species is mediated by the Vancomycin or Van-resistance operon. This operon may be carried chromosomally or extrachromosomally on mobile genetic elements. Vancomycin inhibit cell wall synthesis in gram-positive bacteria. It does this by binding to the terminal D-ala D-ala pentapeptide that is part of the peptidoglycan precursor, thus blocking the trans peptide linkage of cell wall components, and ultimately leading to cell death. The Van operon consists of several genes including genes that encode a variable ligase that mediates the replacement of D-ala D-ala with alternative peptides that have decreased affinity for Vancomycin conferring resistance. One variable ligase is encoded by vanA, a plasmid-borne gene that confers high-level resistance to Vancomycin by replacing D-ala D-ala with D-ala D-lac. The dominant VRE variants of Enterococcus faecium and Enterococcus faecalis worldwide carry the vanA gene. A less common variable ligase encoded by vanB is commonly identified in Enterococcus faecium isolates in Australia, demonstrating that resistance patterns vary geographically. Another variable ligase, vanC, is chromosomally encoded, confers low-level resistance to Vancomycin, and is found most commonly in Enterococcus species other than Enterococcus faecium and Enterococcus faecalis. Emergence of enterococcal species with resistance to second-line agents is concerning. Daptomycin is a semi-synthetic lipopeptide that penetrates the cell wall and inserts its lipid soluble tail into the cytoplasmic membranes of gram-positive organisms disrupting its function and ultimately leading to cell death. Several mutations have been associated with the development of resistance to this drug. Linezolid resistance among Enterococcus species is increasingly reported and most commonly associated with prolonged Linezolid use, although not all cases are associated with prior exposure. This drug class inhibits bacterial protein synthesis by preventing the formation of the 70S ribosomal initiation complex. Resistance is conferred by target modification either from the accumulation of chromosomal mutations or acquisition of a plasmid-mediated CFR gene that was originally described in MRSA isolates. Importantly, Tedizolid, the newest drug of this class, retains activity in the presence of resistance due to the CFR gene. Pneumococcal disease, whether or not due antibiotic-resistant strains, is a major public health problem. However, the emergence of drug resistance further complicates treatment strategies. The CDC estimates that approximately 30% of streptococcal pneumoniae infections are due to drug-resistant isolates resulting in over 1.2 million infections and 7000 deaths in the US every year. Higher incidences of drug-resistant pneumococcus have been reported in other countries. Like MRSA and VRE, the WHO included penicillin non-susceptible Strep pneumoniae as a priority pathogen requiring the development of new agents. Penicillin resistance was first reported in 1978, 26 after penicillin was first used to treat a patient. The slow emergence of penicillin resistance in Strep pneumoniae was the result of chromosomal mutations that alter its penicillin-binding proteins, the target of beta-lactam antibiotics. Beta-lactam resistance among pneumococcus appears to be dose-dependent. In many cases of non-CNS infections, non-susceptibility to beta-lactams is typically overcome by appropriate dosing. However, given the limited CNS penetration of beta-lactams, overcoming resistance in meningitis is challenging. For this reason, a lower breakpoint is used to determine susceptibility for pneumococcus isolated from the CNS compared with those isolated from other sources. This is also why first-line empiric therapy for meningitis includes both Vancomycin and third-generation cephalosporins such as ceftriaxone while waiting culture and susceptibility results. The use of macrolides to treat some pneumococcal infections such as otitis media and pneumonia is limited by increasing resistance. The prevalence of which varies geographically. Macrolides block protein assembly by binding to the 23S subunit of the 50S ribosome. Resistance is conferred by the acquisition of either ermB, whose gene product alters the target site, or mefA, which encodes an efflux pump that expels macrolides from the bacterial cell. The epidemiology of drug-resistant pneumococcus is also impacted by the widespread use of pneumococcal vaccines. Vaccination of children against pneumococcus reduce the number of invasive infections in both children and adults most likely due to reduce nasal carriage of pneumococcal strains in vaccinated children. However, over 90 distinct pneumococcal serotypes have been identified worldwide, and available vaccines target only a portion of these serotypes. After the introduction of pneumococcal vaccines, so-called replacement strains not covered by the vaccine emerged to account for a greater number of pneumococcal infections and some of these exhibit drug resistance. Lipoglycopeptides have a lipophilic side chain linked to a glycopeptide and includes three drugs: Telavancin, Dalbavancin, and Oritavancin. All three exhibit in vitro activity against VRE and MRSA, although the activity of Telavancin and Dalbavancin against VRE is largely limited to vanB strains, which are uncommon. Oritavancin, on the other hand, is active against both vanA and vanB strains of VRE, as well as MRSA. These drugs are approved for use in the US for skin and soft tissue infections, and their use in treating other infections requires further investigation. As we've previously discussed, Linezolid and Tedizolid inhibit bacterial protein synthesis by preventing the formation of the 70S ribosomal initiation complex. These drugs exhibit activity against both MRSA and VRE although resistance to Linezolid is increasing in prevalence. Tedizolid has key structural differences which allow additional target binding site interactions, accounting for its greater potency and retained activity despite Linezolid resistance in some instances. Finally, data regarding the use of combination therapy to treat serious infections due to MRSA and VRE or relapsed infections is emerging. To date, the primary focus has been on a favorable antibacterial interactions between either Vancomycin or Daptomycin in combination with some beta-lactam antibiotics. The precise role of such combination regimens in the clinical setting remains to be determined. In conclusion, antimicrobial resistance in gram-positive organisms presents a serious global public health risk. Slowing the further emergence of resistance depends on the prudent use of antibiotics, and treating infections due to these resistant bugs will depend on the development of more effective antibacterial agents.
