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Wednesday, 20 October 2021

Disinfectant efficacy testing: The need for spiny spores?

There is a difference between US and EN standards in relation to the use of spiny spores. The US standards make no reference, whereas the EN standards require “presence of a high concentration (at least 75 % of spiny spores) of characteristic mature spores, i.e. spiny spores (versus smooth spores)”

Why?

There are three common morphologies of fungal spores:

  • ·         Spores with reticulate structure
  • ·         Spores with spines.
  • ·         Spores with smooth surface.

As some fungi mature the physiology of their spores changes. Younger spores (smoother) are easier to kill than those with spiny or warty surface features. Furthermore, some fungi adapt their spore morphology to the environmental conditions, that is in dryer conditions they are more likely to form smooth or reticulate spores, and in humid conditions they are more likely to form spiny spores. This is because it is optimal in dryer conditions for spores to remain in air currents until a suitable place for germination can be found[i]; whereas, for spiny spores the conditions are generally favourable for growth. Other fungi only display one type of spore ornament, and they have evolved to match certain environmental conditions[ii].

With surface attachment, there are two factors: surface energy and surface topography. In terms of the shape of the spore, main reason is due to surface energy and the contact angle on a surface[1]. When the contact angle is 90° to the surface this results in hydrophobic behaviour and a contact angle of around 160° is classed as called super hydrophobic.


A research group looked at: Fuligo septica var. candida (spine), Metatrichia floriformis

(reticulate), Ceratiomyxa fruticulosa and Licea parasitica (smooth type). With these different forms of spores, the contact angles on a surface were found to be:[iii]

  • ·         63° (± 10.9°) for reticulate spores.
  • ·         16° (±2.35°) for spiny spores.

·         There is no specific contact angle for smooth spores.

This means, on contact with a surface or water molecules, reticulate spores show a small contact area whereas spores with spiny surface show around 50% contact with the medium.

 

The consequence is that spiny spores are most likely to be bound to a surface through greater surface energy, whereas smooth spores are least likely to be bound to a surface. Theoretically, smooth spores pose a greater risk of airborne deposition and spiny spores present a greater risk of surface attachment and treatment with a disinfectant.

There is also a related point that the longer a fungal spore remains in contact with a surface for, the greater potential for the structure to bind with the surface. In addition, depending on the conditions, spiny spores are also more likely to rest and germinate.

Why use spiny spores in disinfectant efficacy testing?

Then reason for favouring spiny spores is:

  • 1.      These are the types of spores most likely to attach to surfaces
  • 2.      These are the types of spores that are theoretically harder to detach from surfaces
  • 3.      These are the types of spores most likely to germinate in a cleanroom environment.


[1] Influence of substratum properties is demonstrated by the relationship between numbers of attached bacteria and substratum work of adhesion for water (WA). This is because the surface energy of a material determines its degree of wettability and, a surface’s affinity for water.



[i] Noblin X, Yang S, Dumais J. 2009 – Surface tension propulsion of fungal spores. J. Exp. Biol. 212:

2835–2843

[ii] Lado C. 2001 – Nomenmyx. A nomenclatural taxabase of myxomycetes. Cuad Trab Flora Micol

Ibér 16: 1–221

[iii] Hoppe T, Schwippert WW 2014 − Hydrophobicity of myxomycete spores: An undescribed aspect

of spore ornamentation. Mycosphere 5(4), 554−559, Doi 10.5943/mycosphere/5/4/8

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (http://www.pharmamicroresources.com/)

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