Sue Dixon, who has died after a short illness, was a formidable microbiologist instrumental in identifying infant formula as the likely cause of a widespread national outbreak of infant gastroenteritis due to salmonella. Her testing techniques which finally identified the source of the bacteria earned her the affectionate nickname "Salmonella Sue".
Dixon was born Suzanne Flora Mander on January 10, 1928, in Malvern, Adelaide. Her father was Arthur Mander, a bootmaker and leather worker from whom Sue got her passion for sport, especially Australian rules. Her mother Flora (nee Oliver) was a seamstress who spent endless hours sewing calisthenics costumes for her two daughters, Maxine and Suzanne. She excelled at school and sport, completing her matric exams a year early yet still finding time to compete in the national calisthenics competition at Ballarat.
![Sue Dixon was affectionately known as "Salmonella Sue".](/web/20170211023215im_/http://www.smh.com.au/content/dam/images/g/u/6/k/j/e/image.related.articleLeadwide.620x349.gu4tyc.png/1486537808437.jpg)
After graduating from Unley High School, Dixon started her career in microbiology as a laboratory assistant cleaning test tubes at the then recently established Institute of Medical and Veterinary Science (IMVS). Dixon was awarded a cadetship by the IMVS to study at Adelaide University where she graduated with a BSc in 1949. From then until 1952 she worked as a bacteriologist at the IMVS. She resigned to start a family with her first husband, Eric Dixon, who she married in 1952. They had two sons, Andrew and Brett. Sue and Eric separated in 1982. In 1985 she married Greg Jacobsen.
After starting her family Dixon rejoined the IMVS in 1960 and assumed responsibility for the national salmonella reference centre established by her mentor and good friend, Dr Nancy Atkinson. From 1967 until her retirement in 1983 she was head of the salmonella reference laboratory and food hygiene laboratory at the IMVS. In this role she developed a comprehensive salmonella reference service in Australia for medical diagnostic, food and environmental testing laboratories.
![Sue Dixon took an innovative approach to the microbiological analysis of foods.](/web/20170211023215im_/http://www.smh.com.au/content/dam/images/g/u/7/0/z/2/image.related.articleLeadwide.620x349.gu4tyc.png/1486537808437.jpg)
Dixon was one of a number of formidable and influential female Australian microbiologists of her generation who played leading roles in transforming the profession of food microbiology in this country. Her contemporaries included Margaret Dick (Kraft Foods, Melbourne), Jenny Taplin (Microbiological Diagnostic Uni, Melbourne University), Dr Barbara Keogh (CSIRO Dairy Research Laboratories, Melbourne), Professor Nancy Millis (Melbourne University) and Dr Phillis Rountree (Royal Prince Alfred Hospital, Sydney). This unique group of women became the touch points for Australian universities, industry and government food testing and research laboratories in training and implementing leading edge clinical and food, water and environmental microbiological analytical techniques.
In 1977, with Taplin, Dixon identified infant formula as the likely cause of a widespread national outbreak of infant gastroenteritis from salmonella bredeney. By studying the epidemiological data from clinical cases they were able to identify a popular brand of infant formula as the most likely source. However, extensive and exhaustive testing of batches of the implicated infant formula by government and food industry laboratories around the country failed to isolate salmonella from any of the samples tested.
It was only through Dixon's forensic and innovative approach to microbiological analysis of foods that the source was ultimately confirmed and traced to contaminated milk powder ingredient. Initially, like all other food testing laboratories, she employed internationally accepted standard methodology for testing foods for salmonella that specified testing five sample lots of 25 grams from each container of infant formula. As this was not yielding any positive results, Dixon progressively increased the size of the samples from 5x25 grams to 5x50 grams, and then to 5x100 grams. Still she was unable to isolate the salmonella.
Convinced that the infant formula was the source of the outbreak Dixon determined that "bucket microbiology" was required and began transferring the entire contents of the 900 gram containers of formula into sterilised stainless steel buckets of nutrient broth in order to massively increase the sensitivity of the test methodology. Using this method Dixon confirmed beyond doubt that milk powder used in the manufacture of the infant formula was the cause of the outbreak.
![Sue Dixon's detective work featured in The Sydney Morning Herald in July 1977.](/web/20170211023215im_/http://www.smh.com.au/content/dam/images/g/u/6/k/j/g/image.related.articleLeadNarrow.300x0.gu4tyc.png/1486537808437.jpg)
Importantly, these findings showed that while the level of salmonella contamination in the infant formula powder was extremely low, once it was reconstituted by mothers and held at warm feeding temperatures the salmonella rapidly multiplied to infective levels. Her work in this area greatly influenced future approaches to investigations of outbreaks of food-borne disease and the methods adopted by regulatory authorities and the Australian dairy industry to prevent salmonella contamination of dried milk products. This development was especially important in meeting food safety requirements for valuable export markets for Australian dried milk products. As a consequence of the national media attention to this investigation she became widely reported and affectionately known as "Salmonella Sue".
The discovery in 1977 of salmonella bredeney in infant formula and the detection around the same time of different serotype of salmonella, s.adelaide, in calcium caseinate from milk powder in a range of dried dairy-based food products such as invalid diet supplements and slimming diets, highlighted the importance of a national salmonella typing system. This led to establishment of a National Salmonella Surveillance Scheme (NSSS). Dixon played a key role in the development of the NSSS, which has been a major tool in the epidemiological investigations of salmonellosis and the early warning to health authorities of food-borne disease from salmonella.
Dixon was also active in the Australian Institute of Food Science and Technology (AIFST). In 1974 she participated in the inaugural AIFST/CSIRO/University of NSW specialist course for the food industry, Food-borne microorganisms of public health significance, where with good humour and diplomacy she taught the teachers some basics in food microbiological techniques. Teachers and participants were startled when the formidable female trio of Dixon, Taplin and Dick turned up as students at the first practical laboratory session and, rather than using the supplied testing equipment, unfurled their own precision microbiological inoculation loops. It was like the "knife" scene from Crocodile Dundee – that's not an inoculating loop, this is an inoculating loop! For subsequent courses (1976 and 1979) Dixon co-authored with George Davey chapters in Food-borne microorganisms of public health significance (the "Green Book"), which has become the standard text in Australia for microbiological food testing laboratories.
In addition to the important role Dixon played in developing national microbiological food standards, improving laboratory testing methodologies and procedures, assisting in the epidemiological investigations of food-borne disease and surveillance of salmonella, she was a role model and mentor to many young aspiring food microbiologists. She was always willing to offer her guidance and wisdom with humour and empathy.
Known for her wicked sense of humour, she was frequently the life of the party at national food microbiological conferences. At one such event in Sydney, Dixon entertained delegates at a post conference drinks session at the famous Albury pub in Paddington by dancing on the bar to an uptempo tune played by the local jazz band. It could have been If You Knew Susie!
She enjoyed a long and happy retirement with husband Greg, playing golf around the world, skiing at Falls Creek, Victoria, in Europe and in the US, as well as tap dancing. All through this period of retirement she maintained an interest in the careers of the many young food microbiologists she mentored.
She leaves sons Andrew and Brett, three grandchildren and one great-grand child.
George Davey AM