Medical Sciences Blog

Infectious Disease – Bacterial Resistance

scidream — Tue, 20 Oct 2009 07:06:00 +0000

Bacterial Resistance

For many years, antibiotics have been critical in the fight against infectious disease caused by bacteria. However, disease-causing microbes that have become resistant to antibiotic drug therapy are becoming an increasing public health problem.

Bacterial resistance against antibiotic occurs when an antibiotic has lost its ability to effectively control or kill bacterial growth; in other words, the bacteria are “resistant” and have developed several ways to resist antibiotics and continue to multiply in the presence of therapeutic levels of an antibiotic.

Increasing use, and misuse, of existing antibiotics in human and veterinary medicine and in agriculture can lead to occurrence of bacterial resistance.

Here, we will learn about 4 types of mechanisms of bacterial resistance to resist antibiotic. They are:

1. Regulation of cell wall permeability

2. Enzymatic inactivation of antibiotic

3. Modification of antibiotic target sites

4. Efflux system

1. Regulation of cell wall permeability

This is the outer cell membrane of gram negative bacteria:

Outer membrane of gram negative bacteria

This mechanism of resistance is only used by gram negative bacteria. Gram negative bacteria have outer membrane filled with porin channels. Mutatios in porins reduce the ability of many beta-lactam antiobiotic to cross the outer membrane of G-ve bacteria to bind at penicillin binding protein (PBP).

Penicillin-binding proteins (PBPs) are a group of proteins that are characterized by their affinity for and binding of penicillin. They are a normal constituent of many bacteria. All beta-lactam antibiotics bind to PBP to have their effect of preventing cell wall construction by the bacterium.

Mechanism of bacterial resistance: Regulation of cell wall permeability

Porins are specific for certain antibiotics. eg. Porin OprD2 allows Imipenem to enter. Hence porins OprD2 mutation in Klebsiella leads to carbapenem resistance.

2. Enzymatic Inactivation of Antibiotic

This is the most prevalent resistance mechanism used by some gram positive and negative bacteria against ß-lactam antibiotic. ß-lactamase, a type of enzyme produced by these bacteria inactivates ß-lactam antibiotic by hydrolysis and breaking of ß-lactam ring of antibiotic.

Beta-lactamase is a broad family of enzyme. There are many types of ß-lactamases e.g. penicillinase, cephalosporinase, ESBL ß-lactamase. Penicillinase is a specific type of β-lactamase, showing specificity for penicillins, again by hydrolysing the beta-lactam ring of penicillins.

Many beta-lactamases are not capable of hydrolysing cephalosporins, but ESBL ß-lactamases have the ability to inactivate cephalosporins. Hence ESBL pathogens will need to be treated with Invanz & Tienam which still have strong activity against ESBL. ESBL is commonly found in Klebsiella pneumoniaa, E. coli, Enterobacteriacea.

ß-lactamases produced by some gram positive and negative bacteria inactivates ß-lactam antibiotic by hydrolysis and breaking of ß-lactam ring of antibiotic.

Mechanism of resistance: Enzymatic inactivation of antibiotic

3. Modification of antibiotic target sites

Modification of PBP by bacteria inhibits the binding of antibiotic to PBP and reduces the affinity of antibiotic binding to PBP. Eg. Streptococcus pneumoniae causes resistant against penicllins by modifying the PBP and reduces the binding of penicillin to it.

Mechanism of resistance: Modification of antibiotic target sites

4. Efflux System

Efflux pumps are transporters localized in the cytoplasmic membrane of gram positive and gram negative bacteria.

Efflux pump is the major mechanism of resistance used by bacteria to resist against antibiotics like Quinolones, Macrolides, Tetracyclines.

Pumps may be specificfor one substrate or may transport a range of structurally dissimilarcompounds (including antibiotics of multiple classes); suchpumps can be associated with multiple drug resistance (MDR). Bacteria combine efflux pump with other mechanisms to increase resistance to multiple antibiotic.

In the prokaryotic kingdom there are five major families ofefflux transporter. Below are the efflux pump used by P. aeruginosa to resist against a broad range of antibiotics. As you can, the most prevalant efflux system, MexAB-OprM causes resistant to Meropenem but not Imipenem.

Bacterial resistance: Efflux pump

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Quick Quiz

Let’s see how much you understand about antibiotics and bacterial resistance so far.

1. Antibiotic and drug testing

Infectious Disease – Classes of Antibiotic

scidream — Mon, 14 Sep 2009 07:03:37 +0000

Classes of Antibiotics

There are many classes of antibiotics.

Penicillin (+ inhibitor)
Cephalosporin (+ inhibitor)
Carbapenem
Quinolone
Aminoglycoside
Anaerobic Agent
Macrolides
Tetracyclines
Others

Infectious Disease – Antibiotic

scidream — Tue, 08 Sep 2009 07:14:22 +0000

Antibiotic

Antibiotic has been used for many years in combacting infectious diseases. Basically what antibiotic does is it kills or inhibits the growth of bacteria.

Mode of Action of Antibiotic

So how does antibiotic work?

Stated in the table below are 4 mode of action (MOA) of antibiotics.

Mechanism of Action	Antibiotics
1. Inhibition of cell wall synthesis These antibiotics disrupt cell wall synthesis only in bacterial cells. Therefore these antibiotics have selective toxicity ie toxic only to the bacteria and is less toxic to human as human cells do not contain cell wall.	Beta-lactam (penicillin, carbapenem, cephalosporin)
2. Inhibition of protein synthesis These antibiotics inhibit protein synthesis on ribosomes in bacteria. Proteins needed for growth & metabolism of bacteria. The selective toxicity of these antibiotics is impaired as bacteria ribosomes resembles mitochondrial ribosomes in human.	Aminoglycoside, tetracyclines, macrolides, chloramphenicol
3. Inhibition of nucleic acid synthesis These antibiotics inhibit DNA synthesis in bacteria and halt cell growth of bacteria.	Quinolone, trimethoprim, nalidixic acid
4. Alteration of cell membrane function Polymixin disrupts cell membranes hence interrupts the exit & entry of cellular materials which leads to disruption of pressure in the bacteria cell and causes lysis. Polymixin is very toxic as there is little difference between human and bacterial cell membrane. When used systemically, there is risk of kidney damage. Hence it is used as the last option.	Polymixin

Mode of action of Antiobiotics: Antibiotics inhibit the growth or kill bacteria by targeting these 4 sites in bacteria cell.

Spectrum of Antibacterial Activity

Spectrum refers to the range & variety of bacteria affected by a particular antibiotic. Antibiotic can be fall into one of these 2 groups:

Broad Spectrum Antibiotic	Narrow Spectrum Antibiotic
Activity against a wide range of gram +ve & -ve, aerobes & anaerobes	Activity against a specific range of bacteria.
TienamInvanz	Vancomycin – MRSAMetronidazole – Anaerobes

If an antibiotic is broad spectrum, it basically means that the particular antibiotic has activity against a broad range of bacteria. On the other hand, metronidazole is reffered to as a narrow spectrum antiobiotic because of its limited activity against mainly anaerobes only. It is reserved for use in clinical setting for treatment against anaerobes related infection.

However, broad or narrow spectrum is a relative term.

Category of Antibiotics

Antibiotics are categorized into these cateogorise based on their ability to kill bacteria.

Bactericidal	Bacteriostatic
Kill bacteria	Inhibit the growth of bacteria without killing the bacteria
Beta-lactam antibiotics, aminoglycosides	Tetracyclines, macrolides
Bactericidal antibiotic is a preferred choice for infections that require prompt eradication of organisms eg. endocarditis and meningitis. Host defenses are relatively ineffective at these sites and the dangers imposed by such infections require prompt eradication of the organisms.	When a bacteriostatic antibiotic is used the duration of therapy must be sufficient to allow cellular and humoral defense mechanisms to eradicate the bacteria.

High concentrations of most bacteriostatic agents are also bactericidal; while low concentrations of bacteriocidal agents are only bacteriostatic.

MIC & MBC

MIC (Minimal Inhibitory Concentration) is the minimal concentration of an antibiotic which inhibits the growth of bacteria.

MBC (Minimal Bactericidal Concentration) refers to the minimal concentration of an antibiotic required to kill bacteria. It is important in immunocompromised patients.

Basic Terminologies of Antibiotic

Let’s take some time to learn more about some basic terminologies of antibiotic in order for us to communicate effective with our physician or microbiologist.

Selective toxicity

Selective toxicity is the ability of an antibiotic to kill/inhibit infecting organisms without damage to host cells. A drug that disrupt the microbial function not found in eukaryotic cells often has a greater selective toxicity.

Potency

Potency is an antibiotic’s ability to eradicate pathogens within its spectrum. The lower the MIC, the more potent the antibiotic is against a particular pathogen. Potency varies with antibiotics, organisms, and strains tested.

Inoculum effect

Inoculum effect is the inability of an antibiotic to remain effective when bacteria concentration increases. There is need to increase dose of antibiotic in serious infections à Increase MIC. Antibiotics with minimal inoculum effect are therapeutically more useful.

Synergistic effect

This term refers to activity of two agents given together exceeds the sum of their individual activities against the pathogen. Synergistic effect can be achieved by combining antibiotics with different mode of action eg. penicillin and aminoglycoside.

Infectious Disease – Nosocomial Pathogens

scidream — Mon, 07 Sep 2009 02:43:54 +0000

Nosocomial Infections

Nosocomial infections are infections resulted from prolonged treatment (usually longer than 48 hours) in a hospital or a healthcare service unit. It is also known as hospital acquired infection or health care associated infections.

Nosocomial infections are transmitted due to the fact that hospitals houses large number of patients with weakened immune system. A patient who is initially admitted to the hospital for urinary tract infection may be infected with nosocomial infection e.g. nosocomial pneumonia due to weakened immune system and prolonged stay in the hospital.

Nosocomial Pathogens

These are some bacteria causing nosocomial infections.

Staphylococci
Enterobacteriaceae
Pseudomonas aeruginosa
Acinetobacter
Bacteriodes
Stenotrophomonas maltophilia

Staphylococci

Cocci = (singular – coccus, plural – cocci) are any bacteria whose overall shape is spherical or nearly spherical.

Main Species	Staphylococcus aureus
Characterisics	Pus producing bacteria
Where	Body surfaces and mucous membranes
Cause	Skin or wound infections, pneumoniae, meningitis, endocarditis, nosocomial infections
Treatment	Gram positive Antibiotic
Treatment for MRSA	Vancomycin, Linezolid, Tygracycline

Staphylococci under the microscope

Enterobacteriaceae

Main Species	Escherichia coliKlebsiella pneumoniaeEnterobacter speciesProteus mirabilisShigella speciesSalmonella species
Where	Normal flora of the intestine
Cause	UTI and intra-abdominal sepsis and septicaemia
Treatment	Gram negative Antibiotic

E coli under the microscope

Pseudomonas

Main Species	Pseudomonas aeruginosa
Characteristics	Serious pathogen in hospital esp. in immunocompromised patientsOpportunistic pathogen
Where	Typically infects the pulmonary tract, urinary tract, burns, wounds, and also causes other blood infections.Colonizer of catheters & ventilators
Cause	Nosocomial infections eg HAP
Treatment	Resistance to wide range of AntibioticSusceptible to anti-pseudomonal antibiotic eg. Tienam, Meropenem, 3rd gen cephalosporins

Pserdomonas aeruginosa under the microscope

Acinetobacter

Main Species	Acinetobacter baumanii
Characterisics	Found in hospital environment, usually ICU or in immunocompormised patientsOpportunistic pathogen
Where	Enter into the body through open wounds, catheters & ventilators.
Cause	Nosocomial infections eg HAP
Treatment	Resistance to wide range of AntibioticSusceptible to Tienam, tygecycline (Tygacil)For multi-resistance acinetobacter – Polymixin B (Very toxic)

Acinetobacter baumannii under the microscope

Bacteroides

Main Species	Bacteroides fragilis
Characteristics	Formation of abscessBacteroides species generally does not cause disease unless they are introduced into deep tissue that are normally sterile eg. Contamination of peritoneal cavity with intestinal contents during surgery/ trauma
Where	Human large gut
Cause	75% of Intra-abdominal infection eg IAI abscess, peritonitis
Treatment	Metronidazole

Stenotrophomonas maltophilia

Main Species	Stenotrophomonas maltophilia
Characteristics	Opportunistic pathogen esp after usage of broad spectrum antibiotic
Where	Aqueous environments: water, urine, or respiratory secretionsFrequently colonizes catheters & ventilators
Cause	Nosocomial infections in immunocompromised patients
Treatment	TMP/SMX (Bactrim), Cefepime (Maxipime), tygecycline (Tygacil)

Infectious Disease – Bacteria Classification

scidream — Fri, 04 Sep 2009 10:53:21 +0000

Bacteria Classification

Bacteria can be classified according to:

Oxygen requirement
Gram Stain

1. Oxygen Requirement

Bacteria can be classified according to their needs for oxygen:

Aerobes	Brow in the presence of oxygen.
Anaerobes	Grow in environment containing no oxygen
Obligate aerobes	Grow only in aerobic environments.
Obligate anaerobes	Do not grow even in the presence of small amounts of oxygen
Facultative aerobes/ facultative anaerobes	Can grow in the presence or absence of oxygen.

2. Gram Stain

Gram staining is a method used to identify two large group of bacteria ie Gram-positive bacteria and Gram-negative bacteria based on the chemical and physical properties of the bacteria cell walls.

The method is invented and named after its inventor, the Danish scientist Hans Christian Gram in 1884.

In this method, bacteria are stained with dye. Gram positive bacteria have a thick layer of peptidoglycan in the cell wall, hence more dye can be absorbed and are stained with purple while Gram-negative bacteria have a thinner layer (10% of cell wall), which stains pink.

Gram Stain

Gram staining is a valuable diagnostic tool in many hospitals and in research field to identify the gram positive and gram negative bacteria. However not all bacteria can be definitively classified by this technique.

In some hospitals, physicians classify bacteria based on these 4 categories.

Bacteria classification in some hospitals

When a physician refers to gram positive bacteria or gram negative bacteria, they are referring to aerobes gram positive and aerobes gram negative. Even though there are gram positive and gram negative anaerobes, they are classified all under anaerobes since there are not many of them. Atypical are generally those bacteria that cannot fall into any of the category.

These are some medically important bacteria according to the above classification.

Examples of gram positive, gram negative, anaerobes and atypical bacteria

Infectious Disease – Bacteria

scidream — Thu, 03 Sep 2009 05:34:29 +0000

Bacteria

Are bacteria our friend or enemy? Let’s get to know more about bacteria.

Bacteria ( singular: bacterium; plural – bacterium) are a large group of unicellular microorganisms. They can be found basically everywhere. They live on our earth, growing in soil, water, and can be found in the live bodies of plants and animals. They live in balance in our body. The vast majority of the bacteria in our body are rendered harmless by the protective effects of the immune system, and a few are beneficial by keeping other potentially harmful bacteria in check.

If bacteria form a parasitic association with other organisms or cause a disease, they are classed as pathogens. Pathogenic bacteria are a major cause of human death and disease and cause infections.

Common Terminologies

Bacteria Structure

Now let us learn more about the structure of bacteria.

This is a typical structure of bacteria:

: Bacteria Structure

Cell wall

The bacteria cell wall is made up of layers of peptidoglycan, a substance that strengthens the bacterial cell wall. Only bacteria have peptidoglycan. Human and animal cells do not contain cell wall.

Hence bacteria cell wall is a specific and good target for antibiotic to kill bacteria. Antibiotics that kill bacteria by only destructing the cell wall of bacteria are less toxic to human as human cells do not contain cell wall.

Cytoplasmic membrane

Both human cells and bacteria contain cytoplasmic membrane. Therefore antibiotic that destructs the cytoplasmic membrance eg. polymixin B are very toxic as there is little difference between human and bacterial cell membrane.

Ribosomes

Ribosomes is the factory that produces protein. The ribosome of bacteria resembles mitochondrial ribosomes in human. Hence antibiotic that target the ribosome may also target the ribosome of human being.

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Knowledge Check

1. Click on this link to play a game on labeling the bacteria stucture: http://www.purposegames.com/game/bacteria-structure-quiz/info

Infectious Disease – Introduction

scidream — Wed, 02 Sep 2009 07:41:28 +0000

Microbiology

Microbiology is the study of microorganism. This subject began with Anton van Leeuwenhoek’s discovery of microorganisms in 1675, using a microscope of his own design.

Microbiology is a broad term which includes virology (Study of virus), mycology (Study of fungi), parasitology (Study of parasite), bacteriology and other branches. A microbiologist is a specialist in microbiology. Here, we will focus on the on the study of bacteria.

Microbiology is researched actively, and the field is advancing continually. We have probably only studied about one percent of all of the microbe species on Earth. Although microbes were first observed over three hundred years ago, the field of microbiology can be said to be in its infancy relative to older biological disciplines such as zoology and botany.

Microorganism

A microorganism or microbe is an organism that is microscopic (usually too small to be seen by the naked human eye).

Microorganisms are very diverse; they include:

What is the Difference between Bacteria and Human Cells?

Human cells and other species of large organism like animal, plants and fungi consist of eukaryotic cells.

Bacteria on the other hand are made up of prokaryotic cells.

The differences between eukaryotic and prokaryotic cells are:

If we compare them in terms of size:

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Quick Quiz

Want to know more about bacteria and other cells? Click on this link to learn more and play some game.

1. Interactive concept in biochemistry