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Tutorial Topics in Infection for the Combined Infection Training Programme

Tutorial Topics in Infection for the Combined Infection Training Programme

Consultant in Clinical Virology

Department of Infection

Barts Health NHS Trust London, UK

Consultant in Medical Microbiology

Department of Infection

Barts Health NHS Trust London, UK

Armine Sefton

Emerita Professor of Clinical Microbiology

Barts and the London School of Medicine and Dentistry

Queen Mary University of London London, UK

3

Great Clarendon Street, Oxford, OX2 6DP, United Kingdom

Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide. Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries

© Oxford University Press 2019

The moral rights of the authors have been asserted

First Edition Published in 2019

Impression: 1

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, by licence or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above

You must not circulate this work in any other form and you must impose this same condition on any acquirer

Published in the United States of America by Oxford University Press 198 Madison Avenue, New York, NY 10016, United States of America

British Library Cataloguing in Publication Data

Data available

Library of Congress Control Number: 2018949670

ISBN 978–0–19–880174–0

Printed and bound by CPI Group (UK) Ltd, Croydon, CR0 4YY

Oxford University Press makes no representation, express or implied, that the drug dosages in this book are correct. Readers must therefore always check the product information and clinical procedures with the most up-to-date published product information and data sheets provided by the manufacturers and the most recent codes of conduct and safety regulations. The authors and the publishers do not accept responsibility or legal liability for any errors in the text or for the misuse or misapplication of material in this work. Except where otherwise stated, drug dosages and recommendations are for the non-pregnant adult who is not breast-feeding

Links to third party websites are provided by Oxford in good faith and for information only. Oxford disclaims any responsibility for the materials contained in any third party website referenced in this work.

This book is dedicated to our colleague and friend Dr Cheuk Yan William Tong, whose knowledge, humility and love for his work continues to inspire us all.

Acknowledgment

Thank you to Dr Maximillian Habibi and Dr Mark Hopkins, who reviewed Dr William Tong’s chapters. We are grateful for your help.

Abbreviations xiii

Contributors xxi

PART 1 BASIC BIOLOGY OF BACTERIA, VIRUSES, FUNGI AND PARASITES; HOST- PATHOGEN RELATIONSHIPS 1

1 The Biology of Bacteria 3

2 The Biology of Viruses 9

3 The Biology of Fungi 19

4 Parasites and Worms 25

5 The Host-Parasite (Microbe) Relationship 35

6 Basic Immunology 44

PART 2 MICROBIOLOGY/ VIROLOGY LABORATORY PRACTICE 55

7 The Use of the Laboratory in the Investigation, Management, and Prevention of Infection 57

8 Bacteriology Diagnostic Methods 64

9 Virological Diagnostic Methods 77

10 Fungal Diagnostics 86

11 Molecular Diagnostics 96

12 Laboratory Quality Control and Accreditation 105

PART 3 HEALTH AND SAFETY 113

13 Biosafety Categorisations and Containment Levels 115

PART 4 PRINCIPLES OF PUBLIC HEALTH IN RELATION TO COMMUNICABLE DISEASES 125

14 Key Communicable Diseases of Public Health Significance and UK Legislation 127

15 Vaccine-Preventable Diseases 136

PART 5 INFECTION PREVENTION AND CONTROL 141

16 Common Organisms Responsible for Healthcare-associated Infection (HCAI) 143

17 The Concept of Chain of Infection and Infection Control Principles 153

18 Antimicrobial Stewardship 161

19 Tools in Infection Prevention and Control 170

20 Sterilization and Decontamination 177

PART 6 IMPORTANT CLINICAL SYNDROMES 185

21 Sepsis—Recognition, Diagnosis, and Management in Adult Patients 187

22 Pyrexia of Unknown Origin (PUO) 193

23 Blood-Borne Viruses 199

24 Tuberculosis and Other Mycobacterial Infections 206

25 Multisystem Infections 213

26 Cardiovascular Infections 220

27 Skin and Soft Tissue Infections 230

28 Bone and Joint Infections 239

29 Upper and Lower Respiratory Tract Infections 245

30 Gastro-intestinal, Hepatic, Pancreatic, and Biliary Infections 251

31 Urinary Tract and Genital Infections including Sexually Transmitted Infections (STIs) 259

32 Infection of the Central Nervous System 266

33 Ocular Infections 279

34 Device-Associated Infections 286

35 Zoonotic Infections 290

36 Exanthemata 297

37 Pregnancy-Associated Infections 305

38 Infections in Neonates and Young Children 313

39 Infections in the Immunocompromised Host 319

40 Post-Infection Syndrome 325

PART 7 UNDERSTANDING USE OF ANTIMICROBIAL AGENTS 329

41 Mechanism of Action of Antimicrobial Agents 331

42 Use of Antimicrobials and Toxicity 339

43 Mechanisms of Antibiotic Resistance 347

44 Detecting Antimicrobial Resistance 354

45 Immunotherapy 363

46 Outpatient Parenteral Antimicrobial Treatment (OPAT) 369

47 Antivirals 375

PART 8 VACCINATION 383

48 Different Types of Vaccines 385

49 Vaccination Schedules 391

50 Vaccination of Specific Groups 397

51 Post-Exposure Prophylaxis 404

PART 9 THE MANAGEMENT OF HIV INFECTION 411

52 Epidemiology and Natural History of HIV 413

53 Laboratory Diagnosis and Monitoring of HIV Infection 422

54 Therapeutic Options for HIV Infection 428

55 Opportunistic Infections in HIV Infection 437

PART 10 TRAVEL AND GEOGRAPHICAL HEALTH 445

56 Geographical Pattern of Infectious Diseases and Infection Prevention for Travellers 447

57 Malaria 454

58 Fever in Returned Travellers 460

Index 467

Abbreviations

+ve positive

-ve negative

A&E Accident and emergency

ACDP Advisory Committee on Dangerous Pathogens

ACT Artemisinin combination therapy

ADEM Acute disseminated encephalomyelitis

AFB Acid fast bacilli

AHA American Heart Association

AMR Antimicrobial resistance

AMS Antimicrobial stewardship

ANTT Aseptic non-touch technique

APC Antigen-presenting cells

API Analytical profile index

ART Antiretroviral therapy

ASOT Antistreptolysin O Titer

AST Antimicrobial susceptibility test

ATLL Adult T cell leukaemia/lymphoma

ATP Adenosine triphosphate

ATS American Thoracic Society

BAL Broncho-alveolar lavage

BBV Blood-borne virus

BCG Bacillus Calmette-Guérin

BDG Beta-D-glucan

BNF British National Formulary

BP Blood pressure

BSAC British Association of Antimicrobial Chemotherapy

BSC Biological safety cabinet

BSE Bovine spongiform encephalopathy

BSI British Standards Institution

CA-MRSA Community-acquired MRSA

CAP Community-acquired pneumonia

cART Combination anti-retroviral therapy

cccDNA Covalent-closed-circular DNA

CCG Clinical Commissioning Groups

CCHF Crimean/Congo haemorrhagic fever

CDC Centers for Disease Control

CDI Clostridium difficile

xiv Abbreviations

cDNA Complementary DNA

CFS Chronic fatigue syndrome

CJD Creutzfeldt-Jakob disease

CL Containment level

CLO Campylobacter-like organism

CMV Cytomegalovirus

CN V Trigeminal nerve

CNS Central nervous system

CNS Coagulase-negative staphylococci

COPD Chronic obstructive pulmonary disease

COSHH Control of Substances Hazardous to Health

CPA Clinical Pathology Accreditation

CPE Cytopathic effect

CPE Carbapenemase-producing Enterobactericeae spp.

Cq Quantification cycle

CQC Care Quality Commission

CRE Carbapenem-resistant enterobacteriacae

CRO Carbapenem-resistant organisms

CRP C-reactive protein

CSF Cerebrospinal fluid

CSF Colony stimulating factor

CT Computed tomography

Ct Crossing threshold

CTLA-4 Cytotoxic T lymphocyte-associated protein 4

CVC Central venous catheter

CVID Combined variable immune deficiency

DAIR Debridement and retention of implant

DALY Disability-adjusted-life-years

DHFR Dihydrofolate reductase

DHR Dihydrorhodamine

DIC Disseminated intravascular coagulation

DIF Direct immunofluorescence

DIPC Director of Infection Prevention & Control

DMSA Dimercaptosuccinic acid

DNA Deoxyribonucleic acid

DoH Department of Health

DTP Diphteria, tetanus, and polio

DVT Deep venous thrombosis

EBV Epstein-Barr Virus

EFI European Federation of Immunogenetics

EIA Enzyme immunoassay

ELISA Enzyme-linked immunosorbent assay

ELISPOT Enzyme-linked immunospot

EM Electron microscopy

EORTC European Organisation for Research and Treatment of Cancer

EPI Expanded Programme on Immunisation

EPP Exposure-prone procedure

ESBL Extended spectrum betalactamase

ESC European Society of Cardiology

ESIMS Electrospray ionization-mass spectrometry

ESR Erythrocyte sedimentation rate

Abbreviations

EVD External ventricular drain

EWD Endoscope washer-disinfector

FBC Full blood count

FITC Fluorescein isothiocyanate

GAS Group A strep

GBS Group B strep

GCS Glasgow Coma Score

GCSF Granulocyte Colony Stimulating Factor

GDH-EIA Glutamate dehydrogenase enzyme immunoassay

GIT Gastro-intestinal tract

GM Galactomannan

GMC General Medical Council

GPEI Global polio eradication initiative

GPEI Glycopeptide-resistant enterococci

GUM Genito-urinary medicine

GVHD Graft versus host disease

H&E Haematoxylin and Eosin

HAART Highly active anti-retroviral therapy

HACEK Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella

HAP Hospital-acquired pneumonia

HBIg Hepatitis B immunoglobin

HBV hepatitis virus B

HCAI Healthcare-associated Infection

HCC Hepatocellular carcinoma

HCCA Alpha-4-cyano-4-hydroxy cinnamic acid

hCMV Human cytomegalovirus

HCV hepatitis virus C

HCW Healthcare worker

HEPA High-efficiency particulate air

HFMD Hand, foot, and mouth disease

hGISA heterogeneous glycopeptide-intermediate S. aureus

Hib Haemophilus influenzae type b

HICPAC Hospital Infection Control Advisory Committee

HII High-impact interventions

HIV Human immunodeficiency virus

HLIU High Level Isolation Unit

HNIg human normal immunoglobulin

HPT Health protection team

HRIg Human rabies immunoglobin

HSE Health and Safety Executive

HSE Herpes simplex encephalitis

HSV Herpes simplex virus

HTA Human Tissue Authority

HTIg Human tetanus immunoglobin

HUS Haemolytic uraemic syndrome

IA Invasive aspergillosis

ICED-IE ICED-infective endocarditis

ICED-LI ICED lead infections

ICP Intracranial pressure

ICPT Infection Control and Prevention Team

ID Infectious diseases

xvi Abbreviations

IDSA Infectious Diseases Society of America

IE Infective endocarditis

IFD Invasive fungal disease

IFU Instructions for use

IgG Immunoglobulin G

IgM Immunoglobulin M

IGRA Interferon Gamma Release Assay

IPC Infection prevention and control

IRIS Immune reconstitution inflammatory syndrome

ISAGA Immunosorbent agglutination assay

ISO International Standards Organization

ITN Insecticide-treated bed nets

ITP Idiopathic thrombyocytopenic purpura

IUGR Intrauterine growth restriction

IV Intravenous(ly)

IVDMDD In Vitro Diagnostic Medical Device Directive

IVDU Intravenous drug use

IVIg Intravenous immunoglobulin

JCVI Joint Committee on Vaccines and Immunisation

KOH Potassium hydroxide

KPC Klebsiella pneumoniae carbapenemase

KS Kaposi’s sarcoma

LAIV Live attenuated intranasal influenza vaccine

LDH Lactate dehydrogenase

LDT Laboratory developed test

LFT Liver function test

LP Lumbar puncture

LPS Lipopolysaccharide

LRINEC Laboratory Risk Indicator for Necrotizing Fasciitis

LTBI Latent TB infection

MAC Membrane attack complex

MAG3 Mercapto acetyl tri-glycine

MALDI-ToF Matrix-assisted laser desorption/ionization-Time of Flight

MAP Mean arterial pressure

MASPs Mannose-binding lectin-associated serine proteases

MAT Microscopic agglutination test

MBL Mannose-binding lectin

MBL Metallo-beta-lactamases

MC&S Microscopy, culture, and sensitivity

MDR Multi-drug resistant

MDR-TB Multidrug-resistant tuberculosis

MDT Multidisciplinary team

MHC Major histocompatibility complex

MHRA Medicines and Healthcare Products Regulatory Authority

MIC Minimum inhibitory concentration

MLSB Macrolides, lincosamides (clindamycin), and streptogramin B

MMR Measles, mumps, rubella

mRNA Messenger RNA

MRSA Meticillin-resistant Staphylococcus aureus

MS Mass spectrometry

MSM Men who have sex with men

MSU Midstream specimen of urine

MTB Mycobacterium tuberculosis

NAAT Nucleic acid amplifying test

NaTHaC National Travel Health Network and Centre

NBT Nitroblue-tetrazolium

NEC Necrotizing enterocolitis

NETs Neutrophil extracellular traps

NEWS National early warning score

NGS Next-generation sequencing

NICE National Institute for Health and Care Excellence

NNRTI Non-nucleoside reverse transcriptase inhibitor

NPV Negative predictive value

NRTI Nucleoside reverse transcriptase inhibitors

NTM Nontuberculous mycobacteria

NVE Native valve endocarditis

OCT Outbreak control team

OI Opportunistic infection

OLM Ocular larva migrans

OPAT Outpatient parenteral antimicrobial treatment

OPV Oral polio vaccine

PAMPs Pathogen-associated molecular patterns

PAN Polyarthritis nodosa

PAS Periodic acid shift

PBP2a Penicillin-binding protein

PCP Pneumocystis jiroveci pneumonia

PCR Polymerase chain reaction

PD Pharamcodynamic

PD-1 Programmed cell death protein 1

PEG Polyethylene glycol

PEP Postexposure prophylaxis

PET Positron emission tomography

PHI Primary HIV infection

PHT Public health team

PI Protease inhibitor

PICC Peripherally inserted central catheter

PII Periods of increased incidence

PIR Post-infection review

PIS Post-infection syndrome

PJI Prosthetic joint infection

PK Pharamcokinetic

PLHIV People living with HIV

PML Progressive multifocal leukoencephalopathy

PMR Polymyalgia rheumatica

PMTCT Prevention of mother-to-child transmission

PNA FISH Peptide nucleic acid fluorescence in-situ hybridization

PPD Purified protein derivative

PPE Personal protective equipment

PPV Positive predictive value

PPV23 Pneumococcal polysaccharide vaccine

PR Rectal examination

PrEP Pre-exposure prophylaxis

Abbreviations

PRRs Pathogen recognition receptors

PTFE Teflon

PTLD Post-transplant lymphoproliferative disease

PUO Pyrexia of unknown origin

PVE Prosthetic valve endocarditis

PVL Panton-Valentine leucocidin

PWID People who inject drugs

QA Quality assurance

QC Quality control

QMS Quality management system

qSOFA Quick Sequential Organ Failure Assessment

RA Rheumatoid arthritis

RBC Red blood cells

RCA Root cause analysis

rCTB Recombinant cholera toxin B

RDT Rapid diagnostic test

RIPL Rare and Imported Pathogens Laboratory

RITA Recent infection testing algorithm

RMP Registered medical practitioner

RNA Ribonucleic acid

RPR Rapid plasma reagin

RR Respiratory rate

RSV Respiratory syncytial virus (or bronchiolitis)

RT Reverse transcriptase

S Svedberg units

S/CO Signal-to-cutoff ratio

SAPO Specified Animal Pathogens Order

SARS Severe Acute Respiratory Syndrome

SCT Stem cell transplant

SD Standard deviation

SDGs Sustainable Development Goals

SFTS Severe fever with thrombocytopaenia syndrome

SIGN Scottish Intercollegiate Guidelines Network

SIRS Systemic Inflammatory Response Syndrome

SIV Simian immunodeficiency virus

SLE Systemic lupus erythematosus

SNHL Sensorineural hearing loss

SOFA Sequential Organ Failure Assessment

SOP Standard operating procedure

SPE Streptococcal pyrogenic exotoxin

SSI Surgical site infection

SSPE Subacute sclerosing panencephalitis

SSTI Skin and soft tissue infection

SUI Serious untoward incident

SVR Sustained virological response

TB Tuberculosis

TBE Tick-borne encephalitis

TCR T cell receptor

Th T-helper

TOE Transoesophageal echocardiogram

ToF Time of flight

Abbreviations

TORCH Toxoplasma, rubella, cytomegalovirus, and herpes simplex virus

TPPA Treponema pallidum particle agglutination

TSI Triple sugar iron

TSS Toxic shock syndrome

U&E Urea and electrolytes

UKAS UK Accreditation Service

ULN Upper limit of normal

URT Upper respiratory tract

UTI Urinary tract infection

UV Ultraviolet

VAP Ventilator-associated pneumonia

VDRL Venereal disease research laboratory

VFR Visiting friends and relatives

VHF Viral haemorrhagic fever

VIP Visual Infusion Phlebitis

VISA Vancomycin-intermediate Staphylococcus aureus

VP Ventriculoperitoneal

VRE Vancomycin-resistant enterococci

VZIG Varicella-Zoster immunoglobulin

VZV Varicella-zoster virus

WBC White blood cells

WCC White cell count

XDR-TB Extensively drug-resistant TB

XLD Xylose lysine deoxycholate

ZN Ziehl-Neelsen

Contributors

Fatima Ahmad, Biomedical Scientist, Department of Infection, Barts Health NHS Trust, London, UK

Sarfaraz Ameen, Anti-Microbial Pharmacist, Barts Health NHS Trust, London, UK

Gurtan Atamturk, Biomedical Scientist, Department of Infection, Barts Health NHS Trust, London, UK

Zahir O. E. Babiker, Assistant Professor and Consultant in Infectious Diseases, College of Medicine and Health Sciences, United Arab Emirates University, Al Ain, United Arab Emirates

Marina Basarab, Consultant, Department of Infection, Barts Health NHS Trust, London, UK

Ruaridh Buchanan, Specialist Registrar in Microbiology and Infectious Diseases, Barts Health NHS Trust, London, UK

Duncan Clark, School of Life Sciences, University of Glasgow, Glasgow, UK

Martina N. Cummins, Clinical Director Infection Prevention and Control, Barts Health NHS Trust, London, UK

Subathira Dakshina, Specialist Registrar in Genitourinary Medicine, Barts Health NHS Trust, London, UK

Satya Das, Consultant, Barts Health NHS Trust, London, UK

Jayshree Dave, Consultant Microbiologist, HPA Specialist Microbiology Network (SMN), Public Health England

Heather Dolphin, Chief Biomedical Scientist, Barts Health NHS Trust, London, UK

Rohma Ghani, Specialty Trainee, Infectious Diseases, Barts Health NHS Trust, London, UK

Jennifer Henderson, Biomedical Scientist, Department of Infection, Barts Health NHS Trust, London, UK

Desmond Hsu, Specialist Registrar in Microbiology, Barts Health NHS Trust, London, UK

Mark Hopkins, Consultant Clinical Scientist, Department of Infection, Barts Health NHS Trust, London, UK

George Jacob, Consultant in Medical Microbiology, Royal Berkshire NHS Foundation Trust, Berkshire, UK

Angelina Jayakumar, Specialist Registrar in Infectious Diseases, Barts Health NHS Trust, London, UK

Anna Jeffery-Smith, Infectious Diseases and Virology Registrar, Barts Health NHS Trust, London, UK

Steve Kempley, Clinical Senior Lecturer in Paediatrics, Blizard Institute, Barts and The London, School of Medicine and Dentistry, London, UK

Palwasha Khan, Clinical Epidemiologist, Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, UK; Interactive Research and Development, Karachi, Pakistan

Jonathan Lambourne, Consultant in Infectious Diseases, Department of Infection, Barts Health NHS Trust, London, UK

Tim Linehan, Biomedical Scientist, Department of Infection, Barts Health NHS Trust, London, UK

Rohini J. Manuel, Consultant Medical Microbiologist, National Infection Service, Public Health England, London, UK

Sylvia Martin, Decontamination Lead, Barts Health NHS Trust, London, UK

Mark Melzer, Consultant in Microbiology and Infectious Diseases, Department of Infection, Barts Health NHS Trust, London, UK

Michael Millar, Consultant, Department of Infection, Barts Health NHS Trust, London, UK

Caoimhe NicFhogartaigh, Consultant in Infectious Diseases and Microbiology, Barts Health NHS Trust, London, UK

Anthony R. Oliver, Programme Manager at CliniSys, Chertsey, UK; Formerly Advanced Practitioner, Virology, Barts Health NHS Trust, UK

Chloe Orkin, Consultant Physician, Barts Health NHS Trust, London, UK

Sarah Parry, Specialist Registrar in Genitourinary Medicine and HIV, Department of Infection and Immunity, Barts Health NHS Trust, UK

Lynette Phee, Clinical Research Fellow, Blizard Institute, Queen Mary University of London; Specialist Registrar in Medical Microbiology and Virology, Barts Health NHS Trust, London, UK

Anna Riddell, Specialist registrar in Infectious Diseases and Virology, Barts Health NHS Trust, London, UK

Caryn Rosmarin, Consultant in Medical Microbiology, Department of Infection, Barts Health NHS Trust, London, UK

Marta Gonzalaz Sanz, Specialist Registar in Medical Microbiology, University College London Hospitals, Barts Health NHS Trust, London, UK

Shila Seaton, Bacteriology Scheme Manager, UK NEQAS for Microbiology, National Infection Service, PHE, Colindale, London, UK

Armine Sefton, Emerita Professor of Clinical Microbiology, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK

Gee Yen Shin, Consultant Virologist, Public Health Laboratory London, Public Health England, London, UK

Stephanie J. Smith, Specialist Registrar in Medical Microbiology, Department of Infection, Barts Health NHS Trust, London, UK

Sherine Thomas, Consultant in Infectious Diseases, Department of Infection, Barts Health NHS Trust, London, UK

Simon Tiberi, Consultant, Department of Infection, Barts Health NHS Trust, London, UK

Cheuk Yan William Tong †, Consultant in Clinical Virology, Department of Infection, Barts Health NHS Trust, London, UK

Robert Serafino Wani, Consultant in Infectious Diseases and Microbiology, Department of Infection, Barts Health NHS Trust, Royal London Hospital, London, UK

David Wareham, Clinical Senior Lecturer/Honorary Consultant in Microbiology, Queen Mary University of London, Barts Health NHS Trust, London, UK

Mark Wilks, Clinical Scientist, Department of Microbiology, Barts Health NHS Trust, London, UK

Elizabeth Williams, Consultant in Sexual Health and HIV, Homerton Sexual Health Services, Homerton University Hospital NHS Foundation Trust, UK

Emily Zinser, Specialist Registrar in Immunology, Department of Immunology, Barts Health NHS Trust, London, UK

PART

BASIC BIOLOGY OF BACTERIA, VIRUSES, FUNGI, AND PARASITES; HOST-

PATHOGEN RELATIONSHIPS

The biology of bacteria

CONTENTS

1.1 Organisms that cause infection in man 3

1.2 What determines pathogen, pathogenicity, and virulence? 3

1.3 The basic structure of bacteria 3

1.4 Other basic ways of categorizing bacteria 6

1.5 How bacteria multiply 6

1.6 DNA exchange between bacteria 6

1.7 What we need to know about each pathogen we study 6

1.8 Assessment questions 7

1.9 Answers and discussion 7

1.1 Organisms that cause infection in man

Bacterial infections and infestations of man can be caused by both microbes and non-microbes. Microbes include bacteria, viruses, fungi, and protozoa. Non-microbes include worms, insects, and arachnids. This chapter concentrates on the basic biology of bacteria.

1.2 What determines pathogen, pathogenicity, and virulence?

A pathogen is an organism that is able to cause disease in its host and the pathogenicity of any organism is its ability to produce disease. Microbes express their pathogenicity by means of their virulence. The virulence of any pathogen is determined by any of its structural, biochemical, or genetic features that enable it to cause disease in the host.

The relationship between a host and a potential pathogen is non-static; the likelihood of any pathogen causing disease in its host depends both on the virulence of the pathogen and the degree of resistance or susceptibility of the host, due mainly to the effectiveness of the host’s defence mechanisms. Two of the main factors influencing a bacteria’s pathogenicity are its ability to invade and it ability to produce toxins—either exotoxins or endotoxins.

1.3 The basic structure of bacteria

Bacteria are unicellular prokaryotic micro-organisms, unlike human cells, which are eukaryotic. Fungi, protozoa, helminths, and arthropods are also eukaryotic. Prokaryotic organisms contain both DNA and RNA, but their genetic material exists unbound in the cytoplasm of the cell as, unlike eukaryotic cells, they have no nuclear membrane. Sometimes bacteria contain additional smaller circular DNA molecules, called plasmids.

The main features of a bacterium are the cell wall, cytoplasm, and cell membrane. However, some bacteria have additional features such as spores, capsules, fimbriae (pili), and flagellae. The

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