Acidophilus is one of the more well-known species of bacteria and belongs to the Lactobacillus group, which is one of the primary species of probiotic bacteria found in the human gut microbiome. Lactobacillus is mostly situated in the small intestine.

Acne is a skin condition that affects many people and symptoms include spots and oily skin. Acne and spots are caused by inflammation of the sebaceous glands and have been linked to gut dysbiosis.

Acne vulgaris is a skin condition that affects many people at some point during their lifetime. Acne presents as oily skin with either comedones (blackheads or whiteheads), pustules, nodules or cysts. Acne is caused by fluctuations in sex hormones that result in inflammation of the sebaceous glands. Gut dysbiosis has been linked to this condition.

Acne conglobata is a severe form of acne that involves a large number of inflamed inter-connected nodules. It is most typically suffered by males who undergo steroid or testosterone treatment.

Acne mechanica is a ‘contact’ condition that occurs often as a result of heat, friction or pressure against the skin. Athletes are most likely to suffer this condition from wearing sports clothing and helmets.

Aerobic is defined as relating to, involving or requiring oxygen. Aerobic bacteria require oxygen for respiration. They need oxygen to survive, reproduce and grow. There are more aerobic bacteria than anaerobic bacteria in the human body.

Aerobic bacteria need oxygen to survive, reproduce and grow. They are more aerobic bacteria than anaerobic bacteria in our bodies.

Aetiology is derived from Greek and means ‘giving a reason for’. It is used in healthcare to refer to the factors or causes that might have led to the current disease.

Anaerobic is defined as without oxygen. Anaerobic bacteria can survive, reproduce and grow without oxygen and can produce energy in an environment that does not contain oxygen.

Antibiotic-associated diarrhoea, or ADD, refers to diarrhoea that occurs as a result of a course of antibiotics. It occurs in around 5-30% of those taking antibiotics and involves loose, watery stools and fever. It results from the disruption of the normal gut microflora caused by the antibiotics, which destroy the existing gut bacteria and thus allow the overgrowth of bacteria that cause diarrhoea.

Antibiotics are medicines designed to treat or prevent some types of bacterial infections in the body. They work by killing the bacteria or preventing them from reproducing or spreading. They are anti-bacterial and do not work for viral infections. Examples of antibiotics include penicillins and tetracyclines. As they work by killing bacteria, antibiotics destroy both the good and bad bacteria in the gut and affect the healthy balance of our microbiome.

Antibodies are proteins produced by the immune system in response to pathogens (foreign substances or antigens) to stop them from entering the body and causing harm. Also known as immunoglobulins, antibodies recognise the pathogens and work to neutralise them and help to remove them from the body.

An antigen is a foreign compound that causes the immune system to respond and produce antibodies. They include substances produced by viruses, bacteria, pollen grains and large carbohydrates.

Antioxidants are compounds that work in our bodies to decrease or slow oxidation, protecting our cells from free radicals (oxidants) which can cause degeneration to the cells. Oxidation is a chemical reaction that occurs throughout the body in normal everyday metabolism and can produce free radicals, which may damage cells. Examples of antioxidants include flavonoids and polyphenols.

Archaea are microorganisms that are similar to bacteria in size and simplicity of structure but radically different in molecular organisation. They are now believed to constitute an ancient group which is intermediate between bacteria and eukaryotes. Archaea are obligate anaerobes living in environments low in oxygen (e.g. water, soil).

Asthma is a respiratory condition that affects the tubes that carry air in and out of the lungs. It often causes symptoms such as coughing, wheezing and difficulty breathing. It is frequently related to other allergies, such as eczema and hay fever.

Atopy is a predisposition to developing allergies or allergic reactions. It is often hereditary and typically involves eczema (atopic dermatitis), allergic rhinitis (hay fever) or allergic asthma – some atopic people have all three. People with atopy also have a tendency to other allergies, including food allergies.

Autism spectrum disorder (ASD) or autism is a development disorder that involves difficulties with social interaction and communication and restricted and/or repetitive behaviour. Many people with ASD are reported to have gastrointestinal problems. Autism, as it is more commonly known, occurs along a spectrum, meaning that the condition will affect people in different ways and with varying degrees of difficulty. Asperger’s syndrome is a form of autism considered to be at the mild end of the spectrum.

Autoimmune disease (AI) occurs when the immune system mistakenly responds to a normal body part and attacks the body’s own tissues. There are more than 80 AI diseases, including type I diabetes, rheumatoid arthritis and inflammatory bowel disease. There is thought to be a link between AI disease and the gut.