Microbial Classification PDF

Microbial Classification Microorganisms divided either to:  Eukaryotes that contain a membrane bound nucleus.  Prokaryotes that contain no nuclear membrane. Differences between eukaryotic and prokaryotic cells are shown in the following table: Table (1): Comparison between euk...

Microbial Classification Microorganisms divided either to:  Eukaryotes that contain a membrane bound nucleus.  Prokaryotes that contain no nuclear membrane. Differences between eukaryotic and prokaryotic cells are shown in the following table: Table (1): Comparison between eukaryotic and prokaryotic cells Eukaryotes Prokaryotes Cells with true nucleus , Contain Cells with premature nucleus , No nucleolus, Has nuclear membrane nucleolus , No nuclear membrane Chromosome is more than one, Chromosome is a one ball of double twisted DNA threads The cytoplasmic membrane contains The cytoplasmic membrane does not sterol contain sterol except mycoplasma There is no mesosomes There is mesosomes They have 80S ribosome They have 70S ribosome The respiratory system is localized in The respiratory system is localized in mitochondrion cytoplasmic membrane Multiply by mitosis Mitosis is absent, Multiply by binary fission e.g. fungi e.g. bacteria & rickettsia 0 Morphology Of The Bacterial Cell Bacteria are small unicellular prokaryotic organisms with rigid cell wall that multiply by simple binary fission. Bacterial Morphology: Staining properties (Gram stain): According to Gram stain, they are divided into 2 groups: 1. Gram positive: bacteria that resist decolorization by alcohol after application of the primary stain and appear violet in colour under the microscope 2. Gram negative: bacteria that decolorization by alcohol after application of the primary stain and take the counter stain (carbol fuchsin) appeared in colour under the microscope (red in colour) Structure of the Bacterial Cell 1. Cell wall Structure: a rigid structure due to peptidoglycan layer. There is a differences in component between Gram positive and Gram negative bacteria. o Cell wall component in Gram positive bacteria: 1- Peptidoglycan layer: 50-60% of the thickness of the cell wall. 2- Teichoic acid. Antigenic structure o Cell wall component in Gram negative bacteria. 1- Peptidoglycan layer: 5-10% of the thickness of the cell wall 2- Lipoprotein layer 3- Outer membrane: found outside lipoprotein layer 4- The lipopolysaccharide layer: endotoxin (lipid A) + polysaccharide O antigen. 5- Periplasmic space Component Gram + Gram - 1. Peptidoglycan Thick Thin 2. Teichoic acid Present Not present 3. Lipopolysaccharide (LPS) (endotoxin) Not present Present 4. Periplasmic space Not Present Present 5. Outer membrane Not present Present Function of the bacterial cell wall: 1  1. Preservation of the shape of the cell (rigidity). 2. Osmotic in sensitive 3. Differentiation of Gram positive & Gram negative staining reaction 4. Antigenicity: - Teichoic acid in Gram +ve is major surface Ag. - Lipopolysaccharides in Gram-ve is major surface Ag called "O" Ag. 5. Toxicity: Lipid A of lipopolysaccharides of G-ve is the endotoxin 6. Target for action of antibiotics: as penicillin and cephalosporins. Cell wall defective bacteria Mycoplasma: only bacterial with deficient in the cell wall. So they are polymorphic and not destroyed by penicillin and not stained by Gram stain. Protoplasts, Spheroplasts and L form: bacterial cell wall may be lost under the effect of certain environmental conditions e.g. treatment with lysosomes and penicillin. protoplasts from Gram positive cells and spheroplasts from Gram negative cells. L form may be found during active infection under the effect of antibiotics (since resist to antibiotic inhibiting cell wall; cause chronic infection). 2. Cytoplasmic membrane (plasma membrane) Structure: It is semi-permeable double layered structure, composed of phospholipid and protein. Function: 1. Selective permeability & Active transport. 2. Energy production, site of respiration. 3. Excretion of pathogenicity proteins and toxins e.g. IgA protease. Mesosome: irregular convoluted invagination of the cytoplasmic membrane into the cytoplasmic Function: 1. Septal mesosomes are attached to chromosome and involved in cell division 2. Involved in secretion of proteins and active transport 3. The sites of the respiratory enzymes 4. Lateral mesosomes: increase the total surface of the membrane II. Cytoplasmic components: 2 1. Nuclear body: DNA is concentrated in the cytoplasm as a nucleoid, no nuclear membrane or nucleolus. 2. Plasmid: they are extra chromosomal DNA molecule 3. Ribosome: (70 S), 30S and 50S subunit. Function: i. protein synthesis ii. target of some antibiotics as tetracycline & chloramphenicol III. Extracellular structures and appendages 1. Capsule: it is formed in-vivo. Composed of polysaccharides except bacillus anthracis composed of polypeptide. Function: 1. Virulence factor protect against phagocytosis. 2. Antigenic: identification and typing of some bacteria 3. Adherence of bacteria to human tissues 4. Capsular polysaccharides are used in some vaccines as pneumococcal, meningococcal and H. influenza vaccine. 2. Flagella: Filamentous appendages that move the bacteria toward nutrients and other attractants (organ of motility). formed of protein flagellin which is antigenic. Types of flagella: 4 types according to arrangement: 1. mono-trichate, single, at one pole 2. amphi-trichate, two, one at each pole 3. lopho-trichate, group of flagella, at one or both poles 4. peri-trichate, all around the surface Function: It responsible for motility of the organism. 1. Important in pathogenesis, by moving the bacteria 2. They are antigenic ( H Ag ), useful in bacterial identification. 3- Pilli (fimberiae): Short hair like fine surface filamentous appendages, they are Shorter and thinner than flagellae. They are formed of protein, found mainly in Gram negative. There are 2 types : 1- Ordinary or common pili, they are antigenic, have a role in adhesion and are virulence factors. 2- Sex pili (fertility pili), longer thicker than ordinary pili. Has a role in conjugation. 7 Bacterial Endospore Definition: highly resistant resting forms developed by certain gram +ve bacilli as bacillus and clostridium when there are unfavorable environmental conditions for their growth as depletion of nutrients, heat, dryness..etc Sporulation: (mechanism of spore formation): The nuclear material + core+ essential enzymes + thick cortex (thick peptidoglycan) + the spore coat (tough keratin like protein) Spore has no metabolic activity and can remain dormant for years. Germination (vegetation): the conversion of spore into vegetative cell when the environmental conditions become favorable for growth. Medical importance of spores: 1- They are resistant to heating (killed at 121oc) so -------- autoclaved. 2- Highly resistant to chemical and disinfectant due to thick coat of spore (need sporecidal) 3- Can survive for many years in soil. Wound contamination can be infected with tetanus. Marked resistant due to: 1. Thick spore cortex and taught spore coat. 2. Low water content so it resists dryness 3- The spore has a rigid impermeable wall rich in dipicolinic acid and calcium. 4- Low metabolic and enzymatic activity. Bacterial physiology Bacterial growth requirements bacterial growth = Increase in the cell mass Nutrition: 1. Autotrophic bacteria: utilize inorganic sources of carbon (CO2) and nitrogen (ammonium). These are usually free living, non parasitic organisms of no medical importance. 2. Heterotrophic bacteria: require organic sources of carbon and nitrogen as sugar and protein. i.e. pathogenic bacteria Gaseous requirements I. Oxygen: 1. Strict aerobic: grow only in the presence of oxygen; O2 is the only H2 acceptor *contain catalase enzyme, hydrolysis of H2O2. Aerobic bacteria breakdown H2O2 by catalase enzyme and O3 superoxide dismutase enzyme. 2. Obligate Anaerobic: growing only in the absence of O2 e.g. clostridium 8 In presence of O2 two toxic molecules are formed; hydrogen peroxide H2O2 and superoxide radical O3 they are toxic to bacteria. Anaerobic bacteria have no catalase or superoxide dismutase enzyme i.e the presence of O2 ® kills the organism 3. Facultative anaerobic: growing in the presence as well as in absence of O2 *they contain catalase and other H2 acceptors e.g most of pathogenic bacteria 4. Microaerophilic: need very small concentration of O2, contain small amount of catalase enzyme e.g. P acne II. Carbon dioxide: Normal atmospheric conc of CO2 (0.03%) ® is sufficient for growth of many bacteria. High CO2 conc (up to 10%) may needed Factors affect metabolic activity of bacteria. Temperature: *Minimum temperature (10 °C). *Maximum temperature (42°C). *Optimal temperature (37°C). Bacterial Products 1. Bacterial pigments: Endopigment & Exopigment Endopigment Exopigment localized in the bacteria Diffuses outside the bacteria Colour the bacterial colonies Colour the bacterial colonies & the surrounding medium e.g. staphylococcus aureus golden e.g. pseudomonas aeruginosa greenish blue yellow colonies Best developed at room temperature They have a role in bacterial respiration 9 2. Bacterial toxins: Types: exotoxins and endotoxins Table 3: Comparison between bacterial exotoxins and endotoxins Exotoxins Endotoxins 1. Diffusibility Diffusible toxins Bound to the body and released only when the organism disintegrate 2. Nature Protein Lipopolysaccharide 4. Organism producing some Gram +ve &-ve Gram-ve bacteria 5. Toxicity Highly toxic Less toxic 6. Antigenicity Strong antigenic Weak antigenic 7. Specificity Specific in action on cells & Non specific tissues 8. Effect of heat Destroyed Stable (60-80 °C) 9. Effect of formalin Detoxicated : change into Not detoxicated formol toxoid 10 Bacterial Growth & Reproduction Bacterial Growth : increases in number. Bacteria divide by asexual simple binary fission i.e Bacterial Growth Curve A. Lag phase: stage of preparation for multiplication during which the organism adapt itself by synthesis of new enzymes specific for the new medium. This stage is characterized by:  No increase in the number of bacteria and there may be a slight decrease due to death of some inoculated bacteria.  Little or no cell division (the bacteria prepare themselves for active cell division). it varies from few hours in E. coli to several weeks in TB depending on: 1. Type of organism (short in E. Coli and long in T.B). 2. Size of the inoculum (the bigger the inoculum, the shorter the lag phase). 3. Stage from which the bacteria are grown (if taken from logarithmic phase the lag phase will be very short). 4. The more suitable the medium, the shorter the lag phase. This phase corresponds in natural infection (in vivo) to the incubation period of disease. B. Logarithmic phase (Exponential phase): rapid multiplication, regularly increase by time. antibiotics are effective during this phase, as B lactam. In vivo it corresponds to the invasion period of the disease. C. Stationary phase: the rate of division decreases and the rate of death increase, this is due to accumulation of metabolic products and O 2 starvation. The number of bacteria divide is equal to the number of bacteria died (the number remains stationary). In vivo it corresponds to the period of clinical signs and symptoms of the disease. D. Decline phase: the death of bacteria gradually increases, at the end the bacteria are completely died; due to accumulation of toxic waste products & release of lytic enzymes and exhaustion of nutrients and O2. It corresponds to the convalescence stage of the disease. 11 Bacterial Genetics Bacteria are suitable cell for genetic studies 1. Contain haploid number of chromosome, immediate expression of character 2. Ability to grow on inanimate media 3. Large numbers of cells per ml that can be obtained rapidly The bacterial genome: chromosome+ Extra-chromosomal DNA (Plasmids & transposons) DNA Structure: two anti-parallel strands of complementary nucleotides DNA Replication: in semi conservative mechanism by DNA polymerase, so the newly formed strands are identical to the original strands Replication forks formation Leading strand synthesis: in a continuous manner in the 5‘- to- 3‘ direction Lagging strand synthesis: in a fragmented manner (Okasaki fragment) in the 5‘- to- 3‘ direction Gene Expression Gene expression occurs in two separate and successive phases: 1. Transcription: the chromosomal DNA is transcribed to mRNA by RNA polymerase 2. Translation: nucleotide sequence on mRNA is translated into the amino acid sequence forming the desired protein Initiation by mRNA + 30 S subunit at methionine codon AUG. Termination: when one of the nonsense (termination) codons. Extra-chromosomal DNA I. Plasmids: Definition: extra-chromosomal small circular double stranded DNA molecule that can replicate autonomously i.e. multiply independently of the chromosome. Plasmids carry genes that encode toxins or proteins that promote the transfer of the plasmid to other cells but usually do not include genes that are essential for cell growth or replication. Classification: plasmids can be classified according to  Number: *stringent control plasmid: one or few copies of per cell *relaxed control plasmid: 10-200 copies per cell  Transmissibility: *conjugative (transmissible) plasmid: genes (tra) + sex pilus; found in Gram negative bacteria 12 *non conjugative (non-transmissible) plasmid: not contain (tra) gene or sex pilus; found in Gram positive bacteria  Compatibility: *compatible plasmid: A single bacterial cell can harbor different types of plasmids *incompatible plasmid: cannot coexist in the same bacterial host and the introduction of a second plasmid, one or the other will be rapidly lost from the cell.  Prevalence *wide range plasmids: can be present in different types of bacteria and can be transmitted from one species to the other *narrow range plasmids: present in one species and cannot be transmitted to the other Functions of plasmid: 1. Conjugation: sex factor plasmid carry the fertility F factor + gene transfer ----conjugation 2. They code for drug resistance, enzymes and toxins production, 3. Used as vehicle in DNA cloning and recombinant DNA technology. 4- Electrophoresis of plasmid DNA give special pattern for each bacterial strain (plasmid fingerprint), used in epidemiological purpose. II. Transposons (jumping genetic elements) Structure: transposon have a short DNA sequence of inverted repeats (IR) on each end of their DNA which are essential for their integration. These are non-replicating mobile DNA sequences that can move between genetic locus and another in bacterial chromosome or between plasmids and chromosome and vice versa. Transposons do not exist as segments free of the genome but only as segments within the genome. Properties 1. They are not capable of autonomous replication 2. They can be lost 3. Insertion of transposon in a gene, disruption of that gene function Function:  They code for drug resistance, enzymes and toxins production,  They are responsible for the ability of some plasmids to integrate into the chromosome. 13 Bacteriophage A bacteriophage (phage) is a virus that specifically infect bacterial cell. Replication: Bacteriophages may have a lytic cycle or a lysogenic cycle,  Lytic cycle : Release of new phage particles is accomplished by lysis of the bacterial cell wall.  Latent or lysogenic cycle: virus integrate with the host chromosome (prophage) and replicate as a part of the bacterium. can be transmitted to daughter cells at each subsequent cell division, and at later events (such as UV radiation or the presence of certain chemicals) can release it, via the lytic cycle. The bacterium lysogenic bacterium it acquired new characters as:  the ability to produce toxin as C. diphtheria  immunity to infection by another phage  allow specialized transduction viruses that can developed both lytic and lysogenic cycle are known as temprate phage Practical application of phages:  Research tools in genetic study  Vector in DNA recombinant technology  Phage typing  Phage therapy: as in urinary tract infection resistant to antibiotic therapy (phage as natural, self- replicating, self-limiting antibiotics). Genetic variations and Gene Rearrangements Mutation: sudden & stable and inherited change Classification I. Genomic mutation This type is classified according to the number of bases affected into 1. Point mutation a- Base pair substitution: replacement of one base pair by another  transition mutation replace: purine base by purine //or pyrimidine base by pyrimidine  transversion mutation replace: purine by a pyrimidine and vise versa b- Base addition or deletion 14 2. Multisite mutation: (>10 bases), occurs by substitution or insertion or deletion or inversion or duplication II. Functional mutation: Same sense (silent mutation): the mutation gives a different codon of the same amino acid. It has no effect on the gene function (silent mutation) Missense mutation: different amino acid, it may affect function of the protein or not according to the position of the altered amino acid Non sense mutation: formation stop codons (UAG, UAA, UGA); causes chain termination (truncated polypeptide chain). It is a very dangerous type of mutation because this always destroy protein function. Frame- shift mutation: the change occur by addition or deletion of one or two bases, it causes shift in the distal reading frame on the ribosome, incorporation of the wrong amino acids "downstream" from the mutation, it leading to production of an inactive protein. III. Inducible mutation 1. Spontaneous mutation 2. Induced mutation: mutation is induced by mutagens it may be Physical as Ultraviolet rays or Chemical Effects of mutation  Inactivate gene products or modify its activity to perform different function  Loss of certain function. Gene Transfer Genes can be transferred from one bacterial cell to another by three distinct mechanisms: conjugation, transduction, and transformation. Transformation: a naked DNA (few number of genes 1 - 3) can be transferred from donor cell to recipient cell. 1. Natural transformation: Donor cell lysed, one of the two strands is destroyed by nucleases, one strand of this donor DNA is taken up into the recipient cell (competence). 2. Artificial transformation: recipient cell can be forced to incorporate plasmids by treatment with calcium chloride &temperature shock Transduction: transfer of genes via a bacteriophage without cell- to-cell contact. Generalized Specialized Types of phage Lytic (virulent) phage Temperate (lysogenic) phage Replication cycle Lytic cycle Lysogenic cycle Mechanism Error in assembly Error in excision Gene transferred Any genes Only chromosomal genes next to (chromosomal or plasmid) the insertion site of prophage 15 Conjugation: transfer genes from one cell to another by cell-to-cell contact. 1. F pili: F+ cell (donor) transfer F factor to (F-) recipient converting recipient into F+. It occur in G-ve bacteria. F+ + F- = F+ + F+ 2. F Factor: (plasmid) 3. HFR ( high frequency recombinant cells): integrated plasmid into the bacterial chromosome HFR + F- = HFR + recombinant F- GENETIC ENGINEERING Recombinant DNA Technology The process of development of new method to isolate genes (DNA segments) coding for certain protein, and join them together to form new combination is called genetic engineering also it is called recombinant DNA technology, or gene cloning. - separation of DNA fragment by restriction endonucleases. The DNA fragment is carried on a vector which is introduced into a host cell by transformation where it is amplified. Restriction endonucleases: enzymes from bacteria can recognize DNA at specific sequence. Molecular cloning vectors: these are the vehicle used to carry and introduced the foreign DNA fragment into a host cell, e.g plasmid, bacteriophage. Application of recombinant DNA technology: 1. Production of vaccines (Recombinant vaccines) as hepatitis vaccine. 2. Production of many proteins as humanized insulin, interferon, human and animal growth hormone. 3. Production of DNA probes used in molecular diagnosis 4- Gene therapy. Gene therapy Definition: Novel approach to prevent disease by changing the expression of person genes. Target: somatic or germ cell Types: Replacement gene therapy Inhibitory gene therapy Clinical applications: 1. Treatment of genetic disorder 2. Treatment of immunodeficiency 3. Treatment of tumours : Pathogenesis of bacterial infection 16  The pathogenicity: is the ability of organism to cause disease. The pathogenesis: is the initiation of the infectious process. Virulence: Is the degree of pathogenicity of the microbe. According to natural habitat and relation to the host, bacteria are divided into: 1- Saprophytic bacteria that grow on dead tissue. 2- Parasitic bacteria that live in or on host tissue. They are divided into: a- Commensal organisms that live in a balance with the host, does not cause disease (Non pathogen) e.g: normal flora of human body. b- Opportunistic pathogen under certain conditions microbiota (some of normal flora) cause disease, some of this conditions are: 1- Impaired host defense mechanisms (i.e. when the patient is "immunocompromised"). 2- Alteration of the host tissues, e.g Strept veridance (normal inhabitant in the mouth) caused endocarditis when reach blood stream after tooth extraction in rheumatic heart patients. 3- Change in the natural habitat of the organisms, e.g if E. coli leave intestine and reach to urinary tract. c- Pathogenic organisms: can cause disease in previously healthy individual with intact immunological defenses. Infection and Diseases Infection is multiplication of an infectious agent within the body. may be inapparent or asymptomatic. Disease the development of signs and symptoms of disease. A cycle of transmission I- Sources of infection: (1) Human (Patient, Carrier): - Carrier is a apparently healthy person that carry pathogenic organism in his body, secrete it to outside without any signs or symptoms. Carriers are more dangerous than patient because: a. They don`t show manifestation of the disease. b. They contact easily with other persons. c. They are not easily detected, not isolated, not treated. (2) Animals (zoonotic infections). (3) Inanimate sources (soil, water, air). II- Root of transmission: (1) Direct transmission: - Direct respiratory spread via large droplets. - Fecal-oral spread. 17 - Sexual transmission. (2) Vector-borne transmission: Is mediated by arthropods or insects. mechanical OR biological (3) Airborne transmission: Mediated by aerosols suspended in the air for long periods. (4) Zoonosis: Any infection spread from a vertebrate animal to a human. III- Portals of entry of pathogenic bacteria into the body: skin, respiratory, gastrointestinal, genital, and urinary tracts. IV-Multiplication of the parasite within the host: either locally at site of entry or may be spread through tissues, blood, lymphatic to reach other sites. V- Portal of exit from the host: e.g urine, stool, respiratory or genital discharge, or from blood by injections or insects. Host Parasites Relationships The relationship between a host and infectious agents may take one of 4 forms: 1- Colonization: colonizes the host tissue without causing any harmful effect. e.g commensal bacteria in oral cavity. 2- Infection: invades the host tissues + elicits immune response but causes minor tissue damage so that no clinical signs appears (subclinical infection). 3- Infectious disease: invades the host tissues + elicits immune response and causes marked tissue damage so that clinical signs and symptoms appears (clinical infection). 4- Carrier Factors affecting host parasites relationships 1- Factors related to the host: Natural and acquired immunity. 2- Factors related to the microorganism: Pathogenicity &Virulence. Virulence factors include: (1) Mechanisms for colonization (adherence and initial multiplication): - Adhesion to cells of tissue surface by specific surface structures called bacterial adhesins as pilli. (2) Invasion of host cells & tissues:.e.g Collagenase: breaks down collagen. (3) Toxin production: exotoxins and endotoxins. Bacterial toxins may be transported by blood and lymph. (4) Ability to bypass or overcome host defense mechanisms: a – Antiphagocytic Factors: e.g Polysaccharide capsules & Pili of Neisseria gonorrhea. b- Intracellular Pathogenicity: Some bacteria (e.g. M. tuberculosis) live and grow in the phagocytic cells. c- IgA Proteases: that split IgA. 18 Antimicrobial chemotherapy - Bactericidal drugs have a rapid killing action of bacteria, which is irreversible. Examples include penicillins, cephalosporins and aminoglycosides. - Bacteriostatic drugs merely inhibit bacterial multiplication, but do not kill them. The bacteria can grow again when the drug is withdrawn. In this case, host defence mechanisms, such as phagocytosis, are required to kill bacteria. Examples include sulphonamides, tetracyclines and chloramphenicol. - Spectrum of Action of Chemotherapeutics: - Broad-spectrum antibiotics; active against several types of microorganisms, both gram positive and gram negative e.g. tetracyclines, chloramphenicol and ampicillin. - Narrow-spectrum antibiotics are active against one or very few types, e.g. vancomycin is primarily used against certain gram positive cocci i.e. staphylococci and enterococci. Mechanisms of action of antimicrobials: - An ideal antimicrobial agent should have selective toxicity, i.e. it can kill or inhibit the growth of a microorganism in concentrations that are not harmful to the cells of the host. - Several mechanisms are known: 1-Inhibition of Bacterial Cell Wall Synthesis: Due to its unique structure and function, the bacterial cell wall is an ideal point of attack by selective toxic agents e.g. Penicillin, cephalosporins and vancomycin, interfere with cell wall synthesis by inhibit peptidoglycan synthesis. β-lactams e.g. penicillin and cephalosporins, and Vancomycin 2- Inhibition of Bacterial Cytoplasmic Membrane Functions: cause disruption of the cytoplasmic membrane and leakage of cellular proteins and nucleotides leading to cell death e.g. Polymyxins, amphotericin B, and nystatin (These drugs are highly toxic as they have a narrow margin of selective toxicity). 3-Inhibition of Bacterial Protein Synthesis: Several drugs inhibit protein synthesis in bacteria without significantly interfering with protein synthesis in human cells. This selectivity is due to the differences between bacterial and human ribosomal proteins, RNA, and associated enzymes. Bacteria have 70S ribosomes (with 50S and 30S subunits), whereas human cells have 80S ribosomes (with 60S and 40S subunits) e.g. Chloramphenicol, erythromycin, linezolid and streptogramins (quinupristin / dalfopristin) act on 50S subunits, while tetracycline and aminoglycosides (gentamicin and arnikin) act on 30S subunits. 4-ln hibition of Bacterial Nucleic Acid Synthesis: These can act on any of the steps of DNA or RNA replication  Quinolones; inhibit DNA synthesis by blocking DNA gyrase,  Nitrofurantoin; Act through damaging bacterial DNA  Rifampicin; inhibits RNA synthesis by binding to RNA polymerase &  Trimethoprim and sulfonamides; inhibit nucleotide synthesis. 5-Competitive Inhibition: In which the chemotherapeutic agent competes with an essential metabolite for the same enzyme. Para-aminobenzoic acid (PABA) is an essential metabolite for many organisms. 19 They use it as a precursor in folic acid synthesis which is essential for nucleic acid synthesis. Sulphonamides: similar to PAPA essential for folic acid synthesis (competitive inhibition) Trimethoprim: inhibit dihydropholic acid reductase.. Dihydrofolic acid and tetrahdrofolic acid… (purine synthesis) Resistance to Antimicrobial Agents The mechanisms by which the organism develops resistance may be one of the following: 1. Inactivating enzyme production: e.g. (penicillinases enzymes and B lactamase destroy the penicillin) 2. Alteration of permeability to the drug e.g. microorganisms change their permeability to the drug. 3. Alteration of target (receptor) for the drug e.g. microorganisms develop altered structural target for the drug. 4. Alteration of metabolic pathway: e.g. microorganisms develop altered metabolic pathway that bypass the reactions inhibited by the drug. e.g. some bacteria not require PABA but utilize performed folic acid. 5. Alteration of enzyme: e.g. microorganisms develop altered enzyme that can still perform its metabolic function but is much less affected by the drug. Origin of Resistance to Antimicrobial Agents A- Non-genetic Drug Resistance: 1- Metabolic inactivity: effectively only on replicating cells. Non-multiplying organisms are phenotypically resistant to drugs e.g. Tubercle bacilli survive for years in tissues and then-resistance to anti-tuberculous drugs is due in part to their metabolic inactivity (dormancy). 2- Loss of target structure: Protoplasts or L-forms of bacteria are penicillin resistant, having lost their cell wall which is the structural target site of the drug. 3- Bacteria may be walled off within an abscess cavity that the drug cannot penetrate effectively. 4- Intrinsic natural resistance, e.g. mycoplasma are naturally resistant to penicillin because they lack a cell wall. B- genetic Drug Resistance: 1) Chromosomal Resistance (Drug Resistant Mutants): - Spontaneous mutation - Altered target. - Rare. - Need the presence of antibiotic - Selective pressure factor. - Low frequency of transmission. 2) Plasmid Resistance (R factor) and transposons: - Presence of R factor (Plasmid that carry the genes of resistance). - Mainly enzymatic. 20 - No need for selective pressure factor. - High frequency of transmission Antimicrobial prophylaxis 1- Prophylaxis in persons of normal susceptibility exposed to specific pathogen e.g. Prophylaxis from Rheumatic fever by long acting Penicillin. Prophylaxis from meningitis by Rifampicin. 2- Prophylaxis in persons of increased susceptibility e.g.: - Heart diseases. - Respiratory diseases (Chronic). - Recurrent urinary tract infections. - Immunosuppressed host. 3- Surgical prophylaxis Antimicrobial combinations Advantages: 1. Serious infection e.g. Peritonitis , meningitis 2. Mixed or unknown infection.e.g. Polytraumatized patients 3. Chronic infection or prolonged treatment.e.g. T.B 4. Prevention or delay of drug resistance.e.g. T.B Disadvantages: 1- Cost is high. 2. Increased incidence of drug reaction 3. Drug antagonism. 4. Increased incidence of Super infection like fungal infections Mechanisms of Drug synergism: 1- Sequential block of a microbial metabolic pathway by the 2 drugs. combining sulfonamides and trimethoprim blocks two steps in the folic acid synthesis 2- One drug may enhance the uptake of the second drug; one drug may affect cell membrane and facilitate the entry of the second drug. Aminoglycoside + penicillin 3- Drug combination may inhibit the bacterial enzymes that destroy the one drug. combination of amoxicillin with clavulanic acid. Clavulanic acid inhibits β-lactamase enzymes produced by some bacteria, preventing them from breaking down amoxicillin. This allows amoxicillin to remain active and effective against β-lactamase-producing bacteria. 21 22 23