Infectious Disease

• Bacteria are single-celled organisms that come in many shapes and sizes.
• Most bacteria are not harmful.
• Pathogenic bacteria cause infectious diseases.
• Colonising bacteria live and multiply harmlessly without causing disease.
• The microbiome refers to the trillions of bacteria colonising the human body, mostly in the gut.
• The bacteria in the microbiome play many important roles, such as protecting against pathogenic bacteria, synthesising vitamins and interacting with the nervous system.
• Bacteria can be categorised into aerobic and anaerobic, gram-positive and gram-negative, and atypical bacteria.
• Learning where bacteria fall within these categories helps determine effective antibiotics.
• Antibiotics are used to treat infections caused by bacteria.
• Aerobic bacteria require oxygen, whereas anaerobic bacteria do not.
• Antibiotics work in various ways to either stop the reproduction and growth of bacteria (bacteriostatic) or kill the bacteria directly (bactericidal).
• Gram-positive bacteria have a thick peptidoglycan cell wall that stains with crystal violet stain.
• Over-use or inappropriate use of antibiotics can lead to antibiotic resistance and limited treatment options for infections.
• The bacteria in different populations develop resistance to different antibiotics.
• For example, the E. coli in one area of the country might be particularly resistant to trimethoprim, while another area has low levels of trimethoprim resistance.
• It is necessary to have local policies that guide what antibiotics to use in different scenarios.
• Antibiotics with a beta-lactam ring include Penicillin, Carbapenems such as meropenem, and Cephalosporins.
• Antibiotics without a beta-lactam ring include Vancomycin and Teicoplanin.
• Bacteria produce folic acid in a series of steps: Para-aminobenzoic acid (PABA) is directly absorbed across the cell membrane into the cell, then PABA is converted to dihydrofolic acid (DHFA), then tetrahydrofolic acid (THFA), then folic acid.
• Eradication usually involves a combination of chlorhexidine body washes and antibacterial nasal creams.
• ESBL bacteria produce beta-lactamase enzymes that destroy the beta-lactam ring on the antibiotic.
• Antibiotics used to treat MRSA include: Doxycycline, Clindamycin, Vancomycin, Teicoplanin, Linezolid.
• When identified, extra measures are taken to eradicate the MRSA and stop it from spreading.
• Atypical bacteria cannot be cultured in the normal way or detected using a gram stain.
• ESBLs can cause other types of infection, such as pneumonia and septicaemia.
• MRSA arises where there is frequent use of antibiotics, such as in healthcare settings.
• ESBLs tend to be E. coli or Klebsiella and typically cause urinary tract infections.
• The usual treatment options for ESBL infections are Nitrofurantoin, Fosfomycin, Carbapenems (e.g., meropenem or imipenem).
• Atypical pneumonia refers to pneumonia caused by atypical bacteria.
• People are often colonised with MRSA bacteria and have them living harmlessly on their skin and respiratory tract.
• The five causes of atypical pneumonia can be remembered with the “Legions of psittaci” mnemonic: Legions – Legionella pneumophila, Psittaci – Chlamydia psittaci, M – Mycoplasma pneumoniae, C – Chlamydophila pneumoniae, Qs – Q fever (Coxiella burnetti), Methicillin-Resistant Staphylococcus Aureus (MRSA) refers to Staphylococcus aureus bacteria that have become resistant to beta-lactam antibiotics.
• Patients being admitted for surgery or inpatient treatment are screened for MRSA colonisation by taking nose and groin swabs.
• Extended-spectrum beta-lactamase (ESBL) bacteria have developed resistance to beta-lactam antibiotics (e.g., penicillins, cephalosporins and carbapenems).
• When infection develops, it can be hard to treat.
• Antibiotics can be used to disrupt steps along this chain: Sulfamethoxazole blocks the conversion of PABA to DHFA, Trimethoprim blocks the conversion of DHFA to THFA, and Co-trimoxazole is a combination of sulfamethoxazole and trimethoprim.
• Trimethoprim is bacteriostatic, while co-trimoxazole is bactericidal.
• Metronidazole is reduced into its active form in anaerobic bacteria.
• Sepsis is a condition where the body launches a large immune response to an infection, causing systemic inflammation and organ dysfunction.
• Macrophages, lymphocytes and mast cells recognise pathogens.
• These immune cells release vast amounts of cytokines, like interleukins and tumour necrosis factor, that activate the immune system.