Pharmaceutical Microbiology Lecture 5 PDF

MICROBIOLOGY AND IMMUNOLOGY DEP. Fall 2024 - 2025 Pharmaceutical Microbiology PM 502 Lecture 5  Lecture outline What is Antimicrobial Resistance Why antimicrobial resistance is a concern Factors promoting antimicrobial resistance Strategies to contain o...

MICROBIOLOGY AND IMMUNOLOGY DEP. Fall 2024 - 2025 Pharmaceutical Microbiology PM 502 Lecture 5  Lecture outline What is Antimicrobial Resistance Why antimicrobial resistance is a concern Factors promoting antimicrobial resistance Strategies to contain or overcome bacterial resistance Genetic bases about bacterial resistance (Intrinsic and acquired resistance) Mechanisms of resistance to antimicrobials What is secondary infection (Superinfection)  Introduction Throughout history, there has been a battle between human beings and micro-organisms that cause infections and diseases. Evolution of bacterial resistance The observation of Staphylococci spp. that could still grow in the presence of penicillin was the beginning of the era of antimicrobial resistance. What is antimicrobial resistance ? Antimicrobial resistance: is the ability of a microorganism to survive and multiply in the presence of an antimicrobial agent that used to inhibit or kill this particular kind of organism. Why is antimicrobial resistance a concern ? Resistant organisms lead to treatment failure & increased mortality. Resistant bacteria such as Pseudomonas, Streptococci, Staphylococci, enterococci or Klebsiella pneumonia may spread in the community or in health care Threats to go back to institutions (nosocomial infections). the pre-antibiotic era.  Factors promoting antimicrobial resistance Antibiotics sold without medical supervision. Prescription not taken correctly. Exposure to sub-optimal levels of antimicrobial agents. Antibiotics taken for viral infections. Antibiotics used in animal's food and in agriculture to prevent infection and promote growth.  Strategies to overcome bacterial resistance Proposals to reduce the abuse of antibiotics range from educational programs for health workers to requiring justification for prescribing certain types of antibiotics. 1) Physicians have the responsibility for making an accurate diagnosis and prescribing the correct therapy. 2) It is important for the patient to take the correct dosage, by the best route, for the appropriate period. 3) Especially valuable antimicrobials may be restricted in their use to only one or two types of infections. 4) The addition of antimicrobials to animal feeds must be reduced worldwide.  Bacterial Resistance to antibiotics is either: Intrinsic Acquired (Innate) Genetic It occurs in organisms that Methods have never been susceptible to a particular drug. Chromosomal EEExtrachromosomal methods methods Horizontal Mutation Gene transfer ❖May occur by mutations in the chromosome OR by acquisition of genes coding for resistance from an external source (plasmid or transposon)  Intrensic Vs. Acquired Resistance Intrensic Resistance Acquired Resistance ❖ It is the natural (innate) ❖Bacteria which were previously ability of a bacterial species to susceptible become resistant resist activity of a particular after exposure to certain antimicrobial agent through its antibiotic. inherent structural or - It occurs due to mutations or functional characteristics. horizontal gene transfer. ❖ Always chromosomally- ❖Mostly plasmid-mediated but mediated. could be chromosomally- mediated.  Examples of Intrinsic resistance: 1] Obligate anaerobes are considered intrinsically resistant to all aminoglycosides since they lack the electron transport system necessary for drug uptake (their uptake across bacterial cell membranes depends on energy derived from aerobic metabolism). 2] The lack of a cell wall in Mycoplasma makes them intrinsically resistant to β-lactams and all antimicrobials that target the cell wall.  Examples of Intrinsic resistance: 3] All Gram-negative bacteria are intrinsically resistant to vancomycin because of it is too large molecule and it cannot pass through the porin channels in their outer membrane. 4] Many Gram-negative organisms, including Pseudomonas, are intrinsically resistant to macrolides and certain β- lactams because the drugs are too hydrophobic to diffuse through the porin channels in the outer bacterial membrane.  Acquired resistance When a particular microorganism obtains (acquires) the ability to resist the activity of a particular antimicrobial agent to which it was previously susceptible. Unlike intrinsic resistance, traits associated with acquired resistance are found only in some strains or subpopulations of each particular bacterial species. Acquired resistance can result from either: (a) Chromosomal Mutations Any change in a single base pair may lead to a corresponding change in one or more of the amino acids for which it codes, which can then change the enzyme or cell structure that consequently changes the affinity or activity of the antimicrobials. (b) Horizontal gene transfer through the acquisition of foreign resistance genes carried on plasmids or transposons by one of the processes: transformation, transduction or conjugation. The role of Horizontal gene transfer is of more clinical importance concerning transferring bacterial resistance than Mutation. Also, the role of conjugation in the transfer of antibiotic resistance is more significant than transformation and transduction.  Many of the antibiotic resistance genes are carried on plasmids or transposons. ❖ Plasmids: Plasmids are extra chromosomal genetic elements that replicate indepedently in the cytoplasm. Plasmids which carry genes for antibiotic resistance are called (R-plasmids) The r-genes can be readily transferred from R-plasmid to another plasmid or to a chromosome. ❖Tansposons (jumping genes): They are sequences of DNA that is capable of moving from one DNA molecule ( chromosome or plasmid) into a new position within the same or another chromosome or plasmid within a single cell.  Biochemical mechanisms of antibiotic resistance 1) Decreasing membrane permeability to the antibiotic due to alteration of membrane structure. 2) Prevention of drug accumulation in the bacterium due to presence of proteins functioning as Efflux pumps. 3) Production of enzymes that inactivate (destroy) or cause modification (alteration) the drug. 4) Alteration of target site which reduce the binding of antibiotics. 5) Using alternative pathways for metabolic requirements. N.B. A resistant microorganism could resist a single antibiotic by more than one mechanism.  A diagram illustrating some mechanisms of antibiotic resistance 1) Prevention of drug accumulation of the bacterium 2) Drug inactivation or modification Bacteria produce enzymes that either destroy the antimicrobial agent before it reaches its target or modify the drug so that it is no longer recognized by the target. Examples: 1- A classic example is the hydrolytic deactivation of the β- lactam ring in penicillins and cephalosporins by the bacterial enzyme called β-lactamase such as resistance of staphylococi to penicillin. 2- Acetylation of chloramphenicol (by chloramphenicol acetyl transferase (CAT). Gram-negative bacteria (ex. Enterobacteriaceae may produce an acetyl transferase that modifies chloramphenicol so that it is no longer active. 3- Gram-negative or Gram-positive bacteria might resist aminoglycosides by producing adenylating, phosphorylating or acetylating enzymes that modify aminoglycosides to be inactive.  3) Alteration of target site In this mechanism, bacterial cells altered the target of the antimicrobial agent (which is typically a critical enzyme or ribosomal site) in a way that decreases its affinity for the antimicrobial agent. This alteration will lead to change in the structure of the target enzyme or organelle (such as ribosome) without affecting the normal function of this target in the bacterial cell. Examples for target site alteration: 1- Alteration in penicillin-binding protein (PBPs) leading to reduced affinity of β-lactam antibiotics. e.g. MRSA 2- Alteration in vancomycin binding site (d-alanyl-d-alanine) reducing activity of vancomycin. e.g. VRSA and VRE 3-Alterations in subunits of DNA gyrase (Topoisomerase II) reducing activity of fluoroquinolones: many Gram- negative bacteria. 4- Changes in RNA polymerase leading to reduced activity of rifampicin: Mycobacterium tuberculosis.  Alteration of metabolic pathway Example: Many bacteria develop resistance against sulfonamide and trimethoprime through deviation from the usual pattern of folic acid synthesis. Sulphonamide resistance can be developed by overproduction of PABA. Tetracycline Resistance occurs mainly through three mechanisms: Efflux of the antibiotics. Ribosome protection (target site alteration) Modification of the antibiotic. A new active form of tetracycline is 9-glycinyltetracyclines Tigicycline which is active against strains containing tet genes responsible for resistance by efflux and ribosomal protection.  Beta-lactam Resistance occurs mainly through three mechanisms: Inactivation of the antibiotic (beta-lactamases). Target site alteration (MRSA). Reduced permeability.  Suppression and alteration of the microflora by antibacterials Most normal, healthy body surfaces, such as the skin, large intestine, outer openings of the uro-genital tract, and oral cavity, provide numerous habitats for a virtual “garden” of microorganisms. These normal colonists or residents, called the flora or microflora, consist mostly of harmless or beneficial bacteria, but a small number can potentially be pathogens. ❖ Superinfection: If a broad-spectrum antimicrobial is introduced into a host to treat a certain infection, it will destroy microbes regardless of their role in the body, affecting not only the targeted infectious agent but also many others (microbiota) in sites far away from the original infection, thus nutrients will be available for the pathogenic bacteria in our bodies to multiply causing a secondary infection. When this therapy destroys beneficial resident species, resistant microbes that were once in small numbers begin to overgrow and cause disease.  Examples of superinfection: 1) Pseudomembraneous colitis: Caused by Clostridium difficile (which is a pathogenic Gram positive bacteria present normally in the colon). This occurs due to destruction of colon microbiota due to the long intake of penicillins, fluoroquinolones or clindamycin, as a result nutrients will be available for C.difficile to multiply and produce its toxins. (Treated mainly with vancomycin; probiotics could be added to restore gut microbiota and prevent recurrence). N.B. Probiotics are live microorganisms—usually bacteria or yeast—that provide health benefits when consumed in adequate amounts, especially for the digestive system. They’re often called "good" or "friendly" bacteria because they help maintain a healthy balance of gut flora, which is essential for digestion, immunity, and even mental health. 2) Vaginal candidiasis: Using any broad-spectrum antibiotic to treat a urinary tract infection by Escherichia coli would cure the infection, but it will also destroy the lactobacilli which is a normal flora present in the vagina and maintains a protective acidic environment. Therefore, nutrients would be available for Candida albicans (fungi that is also present in normal vaginas) to proliferate and cause a secondary infection with candida. 3) Oral candidiasis = thrush: Candida present in the oral cavity can cause similar superinfections of the oropharynx due to destruction of oral microbiota as a result of the intake of antibiotics for long periods of time. Thrush may extend to oesophagus and the large intestine.  Some clinical situations in which prophylactic antibiotics are indicated: 1- Pretreatment may prevent streptococcal infections in patients with a history of rheumatic heart disease. Patients may require years of treatment. 2- Pretreating of patients undergoing dental extractions or those having implanted prosthetic devices, such as artificial heart valves, to prevent contamination of the prosthetic devices. 3- Treatment prior to most surgical procedures can decrease the incidence of infection afterwards. THANK YOU