Skip to main content
ARS Home » Northeast Area » Wyndmoor, Pennsylvania » Eastern Regional Research Center » Characterization and Interventions for Foodborne Pathogens » Research » Publications at this Location » Publication #300052

Title: Detecting bacteria in food- harder than searching for a needle in a haystack

item Gehring, Andrew

Submitted to: New Food
Publication Type: Popular Publication
Publication Acceptance Date: 11/28/2013
Publication Date: 12/1/2013
Citation: Gehring, A.G. 2013. Detecting bacteria in food- harder than searching for a needle in a haystack. New Food. 16(6):24-28.

Interpretive Summary:

Technical Abstract: Actually, I think it would have been easier to find a needle in a haystack than to locate variables pertaining to that time-honored question and mathematically compute the relative difficulty of detecting bacteria versus searching for aforementioned needle. Perhaps it might be unusual given that I am employed by the US Dept. of Agriculture, but before writing this article, I never realized that there are at least 3 styles of haystacks that varies in volume and weight depending upon the species of grass, drying time, rainfall, packing density, etc.… and as for sewing needles, apparently the debate on which is the most common needle (that one should use for an empirical calculation such as this) is about as mind boggling as figuring out the difference between European versus USA sizing conventions for needles! Easier to find, bacteria or needle?, note, if one thought it an interesting challenge to develop biosensors* for the purpose of detecting 1 of these bacteria in 1 mL (or cm3) of liquid they too would have applied for my job with the USDA back in the mid-1990s. Anyway, the problem first merits some guesstimates-- racking my brain, I recall that typical rod-shaped bacteria such as the 2nd most prevalent Salmonella isolated from the US in 2009 foodborne pathogen Salmonella typhimurium (aka Salmonella enterica subspecies enterica serovar Typhimurium… i.e., a “bug” that causes typhoid like symptoms in infected mice… & can also at the very least cause humans to develop gastrointestinal issues) is about 5 mu (micrometers or 1 millionth of a meter) or 0.0005 cm long and about 1 um (0.0001 cm) wide. Making some assumptions, ignoring close packing (assuming pegs fit nicely into square holes), a volume ratio of 1 bacterium in 1 mL of water is roughly 1 part in 64 billion parts. By comparison, a needle (also assumed to be a rod, but 3 cm in length and 0.1 cm wide) and hay stack with attributes gleaned from a 1931 USDA technical bulletin1; “wild” hay, dried 30-90 days; approximately 600 ft3 per short ton (approximately 19 m**3 per metric ton), and for the sake of simplicity, assumed to be in a non-water shedding square with flat top configuration with a base of 20 ft x 20 ft and height of 12.5 ft (approximately 140 m**3) would have a volume ratio of 1 part in 57 billion parts. So the odds of finding a needle are slightly higher, but then again, a needle can be seen by eye. Two competing teams on an episode of the popular TV show “Myth Busters” ingeniously tackled the needle in haystack problem via 2 approaches. One team used water to separate the denser stainless steel needles (4 in all) from the 10 bales of dried hay. That team succeeded in locating the needles in a more rapid time than the other team that processed their challenge through setting fire to the hay. Unfortunately, bacteria are mostly water and if the water “burns,” the bacteria will surely burn too so neither of these techniques could apply to the bacteria detection conundrum. I wonder if handheld microwave emitters are for sale? For years now, it has been cautioned that a person, particularly young, old, or immune-compromised, is more prone to become sick from ingestion of a single, live pathogenic bacteria known as E. coli O157:H7 [as well as many serotypes of Shiga toxin-producing E. coli or STECs] infamous for the Jack in the Box outbreak of 1993. Live bacteria can multiply in a host and thus cause an infection that can lead to devastating effects such as hemolytic fever, kidney failure, and brain damage. Forgoing pre-enrichment or resuscitation in order to revive/recover potentially injured cells, very small amounts of bacteria are traditionally detected after a growth enrichment period of typically at least overnight on nutrient-laden solid agar in Petri dishes. This technique allows for the visual examination of bacterial colonies for enumeration a