• There has been Ý resistance to antibiotics due to their widespread use. "We currently have the knowledge to prolong their effectiveness, but do we have the will, and the courage, to take the measures necessary to ensure their survival?"

• Ý severity of illness
• newer devices and procedures
• Ineffective infection control and compliance
• Ý use of prophylactic and/or empiric antibiotics

• (1) Ý prevalence of infection caused by naturally resistance species previously of little importance
• Ex. Emergence of gram negative bacilli as hospital pathogens following penicillin treatment; Ex. Enterococcus after widespread cephalosporin use
• (2) Mutational changes in previously susceptible bacterial populations
• Ex. staphylococci resistance to rifampin; resistance to newer fluoroquinolones by many bacteria
• (3) Acquisition of foreign DNA
• Ex. plasmid mediated b-lactamases and vancomycin resistance in enterococci
• (4) Mutation of acquired genes
• Ex. extended spectrum b-lactamases found in Klebsiella pneumoniae
• (5) Regulatory mutations in normal cellular genes
• Ex. multiple antibiotic resistance (MAR) in many gram neg bacilli and the inducible b -lactamases of Pseudomonas aeruginosa, Enterbacter spp. and other gram neg bacilli

 b-lactams – History, Production of New Drugs and Development of Resistant Bugs

• Penicillin drugs were first introduced in the late 1930’s and within 10 years 50% of hospital staphylococcal strains produced b -lactamases which inactivated b-lactams
• b-lactamase resistant drugs Þ Vancomycin was introduced in 1958 methicillin in 1961 and then the cephalosporins
• ampicillin was developed later to treat E. coli infections; shortly after the introduction of ampicillin, ampicillin-resistant E. coli strains were recognized
• Mechanisms of b-lactam Resistance
• (1) Alteration of penicillin binding proteins (PBP)
• (2) Outer membrane proteins
• (3) b-lactamases which break the b -lactam bond (Four classes, A-D)
• Class A. Contains Enzymes TEM, SHV and PC1; Þ constitutively produced but not active against cephalosporins
• TEM is frequently located on plasmids and can be transferred between different strains Þ widespread in gram-neg bacteria such as Haemophilus influenzae, Moraxella catarrhalis and Neisseria gonorrhoeae
• the plasmid containing TEM often confers resistance to trimethoprim-sulfamethaxazole (TMP-SMX) that is mediated by the production of two different enzymes Þ a dihydropterate synthetase (SMX resistance) and a tetrahydrofolate reductase (TMP resistance)
• Integrons – collections of resistance genes frequently found on plasmids in gram-neg bacilli; the mechanism by which dihydopterate synthetase and tetrahydrofolate reductase are transferred
• b-Lactams Resistant to hydrolysis by b-lactamases
• Methicillin, Oxacillin, Naficillin Þ activity restricted to gram-pos bacteria
• Monobactam (Aztreonam) Þ activity restricted to gram-neg bacteria
• Carbapenems (Imepenem, Meropenem) Þ very broad spectrum of activity
• Cephalosporins Þ both gram-pos and gram-neg activity; spectrum depends on generation 

• Methicillin Resistant Staph. Aureus (MRSA)
• unlike penicillin-resistant staphylococci, these organisms do not produce a b -lactamase
• they acquired a novel PBP with a low affinity for virtually all b -lactam antibiotics (expect ampicillin) Þ although ampicillin could still bind MRSA PBP, most MRSA also produced b -lactamase making ampicillin ineffective
• methicillin resistance has not been shown to be transferable in vitro between staphylococci Þ MRSA is spread not by transferring the genetic material between bugs, but by the direct person to person spread of S. aureus
• many MRSA are also resistant to gentamicin Þ many MRSA are now resistant to everything but vancomycin
• unfortunately vancomycin is less active than b -lactams against susceptible staphylococci
• 50% of people colonized with MRSA are still colonized after 40 months and new data suggests that pets may also be carriers
• Enterococci
• enterococci are intrinsically resistant to cephalosporins and are tolerant to the activity of all cell wall active antibiotics Þ they are an important nosocomial pathogen and require multiple antibiotic therapy to treat
• endocarditis treatment requires bactericidal antibiotics Þ enterococcal endocarditis can be treated with a cell wall active antibiotic (ampicillin or vancomycin) combined with an aminoglycoside (streptomycin or gentamicin) resulting in bactericidal activity and a cure rate of 80% (compared to only 40% with penicillin alone)
• enterococci became resistant to gentamicin shortly after staphylococci Þ both classes of bacteria are resistant
• genes responsible for both enterococci and staphylococci resistance to gentamicin are carried on transposons and cause the inactivation of aminoglycosides once they enter the cell Þ cure rates for enterococcal endocarditis expressing resistance to both gentamicin and streptomycin are about 40%
• Klebsiella pneumoniae
• due to widespread cephalosporin use, many are now resistant; can be controlled by ß cephalosporin use
• resistance commonly conferred by mutated variants in the TEM enzyme through only one or two amino acid modifications Þ b -lactamases now capable to hydrolyze cephalosporins
• enzymes commonly found on large transferable plasmids that confer resistance to many antibiotics Þ TMP-SMX resistance was also conferred to K. pneumoniae
• prophylactic use of TMP-SMX for Pneumocystis carinii pneumonia may predispose people for resistant strands
• Ý use of fluoroquinolones which is also causing ß their effectiveness due to Ý resistance
• Streptococci pneumoniae
• remained susceptible to penicillin for many decades but now 33% of pneumococci indicate some level of penicillin resistance and 13 % express high-level penicillin resistance
• often resistant to many antibiotics Þ TMP-SMX (90% resistant), Tetracycline (50%), Erythromycin (50%), Chloramphenicol (40%)
• penicillin resistance is due to the production of low affinity PBP acquired from viridans streptococci
• pneumococcal resistance is due to Ý penicillin use in the outpatient setting
• penicillin does achieve high concentrations in the CSF and in the middle ear and complicates the treatment of meningitis and otitis media
• Haemophilus influenzae
• thirty percent of H. influenza produce b-lactamases and pharmaceutical companies use this fact to promote b -lactamase resistant cephalosporins in the treatment of otitis media. However, only 30% of all otitis media is caused by H. influenza, ß the chance of a b -lactamase producing H. influenza as the cause of otitis media down to 10%. In addition, studies indicate the H. influenza will be undetectable in the middle ear on day 2 of placebo therapy in 50% of cases, reducing the need for cephalosporins down to only 5%. On top of this, most children are vaccinated with the Hib vaccine that prevents H. influenza meningitis. Amoxicillin remains more active against penicillin-resistant pneumococci than most oral cephalosporins and remains the recommended treatment of otitis media.
• Vancomycin resistant Enterococci (VRE)
• VRE are frequently resistant to all clinically available antimicrobial agents
• Enterococcus faecium is the primary bacterium Þ has low virulence and is primarily a problem for immune-compromised patients.
• Major problem in hospitals causing considerable morbidity and Ý cost of hospital care
• Pose a threat of transferring their resistance determinants to more pathogenic species for which vancomycin represents the only therapeutic option, such as MRSA